Editing v0.31:Security design

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{{Quality|Exceptional}}

:''This page is one of several inter-related articles on the broader topic of defending your fortress and your dwarves. '''Security design''' focuses on how to turn the physical layout and architecture of a fort into a defensible whole. For a general overview of the threats that will challenge your fortress and things to consider when preparing a standard defence, see the '''[[defense guide]]'''. For complex traps that are not a minor/optional part of a larger defensive plan (but might be adapted or plugged into one), see '''[[trap design]]'''. For specific advice on how to get your soldiers prepared for any threat, see '''[[military design]]'''.

:::::'''''Editors & Contributors''' - Please see the discussion page before posting.''


==Standard key==

   '''Key:'''
 '''symbol  tile'''
  ·   -  Empty space
  +   -  Constructed floor, or top of wall section from lower level
  '''0'''   -  Isolated wall section
 ╔╦═╗
 ╠╬═╣ -  Connected wall 
 ║║ ║
 ╚╩═╝
  ╬   -  Fortifications
  X   -  Up/down [[stairs]]
  <   -  Up stair
  >   -  Down stair
  ▲   -  Up ramp/slope
  ▼   -  Down ramp/slope
  .   -  natural ground
  ☺   -  dwarf

==General suggestions==
General designs should include suggestions that can be "plugged in" to a part of any typical fortress, and/or can be modified to suit a number of purposes.

Any fortress defenses need to be able to protect your dwarves while outside, whether that's military or civilians.  On the truly labor intensive end, you can fully enclose areas of wilderness you wish to utilize in walls or behind moats with the only access being from within your base.  Hostile creatures, even 'invisible' ones like ambushers, start at map edges and travel across the map - they will only spawn in regions where they can path to a dwarf.  By controlling which areas have access to paths to dwarves, you can force all hostile forces to appear in predictable and limited killing zones and battlefields that you control.  

===Meeting area as defense===

Especially in the very early game, you can use a [[Zone#Meeting_Area|meeting zone]] to attract animals and idle dwarves to a given area. This makes a pretty poor defense in general, but it's not a bad way to create an alarm system against minor threats such as [[thief|thieves]] near your stockpiles, at least until you have something better (which won't be hard).  Remove the zone later, or it attracts idle dwarves and children.  Note that until you designate something else, the site of your wagon (even once deconstructed) is a default meeting area.

===Guard Animals===

Both [[thief|thieves]] and [[ambush]]es are invisible until something bumps into them - a dwarf, a [[caravan]], a wild creature, a [[domestic animal]], anything.  Once this happens (even if it was triggered by a wild [[groundhog]] on the far edge of the map), the game will pause with the appropriate [[announcement]], forcing your attention to the situation - which is nice.  Therefore, it's a common practice to use animals to act as alarm systems, by [[restraint|restrain]]ing or assigning them to a [[pasture]] in entryways.

There are some considerations to good placement of such animals.  If you have a 1- or 2-wide hall, one animal is enough.  If you have a 3-wide hallway, a single pastured animal placed in the middle is still sufficient, or you can restrain two animals, one at each side of the hall.  

 ══════════     R = restraint
 +++1R1++++     1 = area of animal 1
 +++bbb++++     2 = area of animal 2
 +++2R2++++     b = area of both
 ══════════

Either arrangement creates a thief-proof barrier against unannounced intrusion, as there is no combination of locations where an invisible enemy can sneak by without bumping into an animal. Caravans can pass over [[restraint]]s and [[pasture]]s and their contained creatures without problem.  Guard animals can also see hidden enemies one z-level below them, so long as there is no intervening floor, so if space is tight you can also place them above your entranceway.

Unless you're happy losing these animals on a regular basis, you should try to keep them alive.
:* Put them around a corner or behind a U-bend, so archers cannot fire at them from a distance.
:* Don't have them as your "first line of defense"; put them deeper in the entry, behind some traps.
:* Put them inside, so flying creatures have to come down to their level to attack them.
:* Consider using a [[pressure plate]] at the extreme entrance to seal off the hall further down and keep your guard animal(s) safe.  Thieves won't trigger them, but the animals can deal with those - ambushes ''will'' trigger them, and you don't want them getting to your guard animals.

Remember that anything short of a [[megabeast]] is not a good match for an armoured opponent.{{verify}} While watching your tame [[grizzly bear]] or [[alligator]] tear a thief apart has an amusement value, watching the goblin maceman send them flying across the map, mangled and dying, has less.

Hunting Animals (trained at [[kennel]]s) have better observation (sight range) than their regular or war counterparts.

===Defending the edge===
You're not allowed to wall within five squares of the edge of the map... but this rule has more loopholes than the US federal income tax code.  Until more versatile attackers emerge, it is not clear where effective play ends and exploit begins.  ''(Note: we disclaim any responsibility for damage involving [[harpy|harpies]] and skeletal [[giant eagle]]s)''

* To start with, you can channel the '''''second''''' square from the edge.  In the days of [[wagon]]s, this used to block the appearance of trade caravans or their movement along the edge of the map.  If barriers are used to prevent a Trade Depot near the edge of the map from being accessed from any other direction, caravans will be forced to appear in the un-channeled or bridged section of the edge.  Your depot can be ready with stockpiles of favored trade goods, offset behind a wall to protect from archers a few squares away.  
* You can also build drawbridges all the way up to the edge.  A long, skinny, raised bridge is effectively a wall; however, it looks the same whether it's open or closed.
* If you channel to the second square from the edge where the '''''edge square''''' contains a tree then you have an impassible barrier while the tree survives.
* You can with some work use [[Obsidian]] casting to wall to the map edge

Train up diggers in soft soil and you can surround most of the map with a moat by the time the first migrants arrive.  Be very, very wary of cave-ins, especially on highly sloped diagonal terrain - note that a downward ramp does not support adjacent floor tiles, and no tiles are supported diagonally.

Diplomats have a strange habit of appearing well inside the moat, but need to be allowed out when finished.  ''[Note: On one 6x7 map [[horse]]s and other animals were also found to appear one embark unit (48 squares) left and up from the lower right corner, inside or atop the walls of a 5x5 doorless enclosure.  Defend all leaks...]''

==== Design considerations ====

* The moat should be designed to prevent entry except by falling and exit except by climbing, from both sides.  (Otherwise inside and outside forces might be tempted to shake hands from adjacent squares, with much annoyance)  Despite an abundance of giant corkscrews, grates, ballista bolts, etc., no one has ever invented the ladder, so this keeps anyone from entering or leaving the rest of the map.
* The moat should be dry, because sooner or later you will be tempted to let someone visit the edge to loot goblins or hunt varmints, and next thing you know your Legendary Weaponsmith who outpaces all your smelters will be whiling away his time carrying a leather thong to a stockpile when he runs into a groundhog and decides to react by jumping into the moat and holding his breath beneath the shallow waters until he drowns.  (As always, the notice that he has drowned is the first you'll hear of it)
* The moat doesn't actually ''need'' to be adjacent to the edge of the map except when conserving valuable surface terrain (such as [[tree|trees]] on a map that is mostly rock).  It is easier to free trapped miners when they can dig further outward, and placing the moat on the sixth or further square in from the edge allows further modification with floodgates, walls, and doors.  Any [[channel]]ing permanently changes the dug-out tile to "Light Above Ground", which restricts these features from tiles near the edge even if floors are later constructed to close the space.
* Because migrants might turn up near wild animals or be followed closely by [[goblin|goblins]], it is nice to wall off the last square in shorter segments.  Each one or two segments are served by a separate lever bridge.  This can be done by:
** Natural barriers.  The map edge is mostly continuous ramp, but occasionally a break appears on an uneven surface, by a river channel, etc.
** Trees.  If left intact they separate any fertile patches into many small segments.
** Floors.  Although you can't directly Remove Stairs/Ramps at the edge, building a single square of floor on an up-ramp at the edge will destroy that up-ramp (and the down-ramp above it) and block movement around the edge.  Building a square of floor on a down-ramp and then removing it creates a [[one-way]] path.<sup>was a fix exploit, verify.</sup>
** You can build drawbridges *along* the edge and raise them.  Combined with a channel right next to the drawbridge, this can completely obstruct passage of anything which can't destroy the bridge.
** You can build up ramps at the edge, which may disrupt passage?<sup>Needs testing</sup>
** Fortifications carved into the outer edge rock the next layer down?  It may be possible to carve fortifications all the way around the edge of a rocky map, allowing entrance only onto designated bridges surrounded by moat and with a steep drop beneath, with some sized appropriately to admit siegers only and one other sized for a trade wagon.  In this way combat can be reduced to a simple thumbs up/thumbs down decision at the lever.<sup>Probably not.</sup>
* Migrants, thieves, and sieges turn up all around the map, and can be allowed in by remote controlled bridges.  (Doors will not hold back [[building destroyer|building destroyers]], and remote [[lever]] control is needed because other gates can be "taken by invaders" and become non-lockable) Invaders can be allowed in by small groups and fought if desired, or preferably admitted into underground zigzags with a door waiting to be locked at the far end once they get close to it.  If most of the invaders can be trapped inside such spaces, the remainder will stand and be wiped out completely without retreating.

===Simple 5x5 Archer's Tower===

Build a [[tower]] specifically to post archers on, possibly away from your main defenses. This lets you open fire before the enemy approaches your gates. A pillbox can be attached to your walls, or separate, so that the only access is from tunnels below. These tunnels can stretch across the map, and only need be 1-tile big if no regular traffic is expected. Construct [[wall]]s up to the second or third floor and then carve fortifications into them, so your dwarves can fire out.  For extra usefulness, build a [[barracks]], [[archery target]], [[food]] [[stockpile]], [[well]] and/or [[dining room]] in or near the tower. Add a door or hatch to lock them in.

NOTE: It is better to construct walls and then carve fortifications into them, than to construct fortifications. The reason for this is that constructed fortifications do not provide a walkable tile on the layer above them, as walls do. Walls cannot be constructed without access from one of the four compass directions. Because of this, if you were to construct fortifications, when you progress to the level above, you would need to build a walkable path to the corner tiles. This path would need to be deconstructed before the wall or fortification could be built. Once deconstructed, the building material will drop onto the tile below with the fortification, trapping it until the fortification is deconstructed.

As discussed step-by-step in the article on [[mega construction]], this particular design is about as basic as it gets.  As shown, it assumes entry from an underground tunnel, but a door or drawbridge (with moat!?) could easily be added, or even access via a protected sky-bridge. 

When placing multiple towers, know that crossbows have a range of 20 tiles, so, depending on whether you want overlapping fire or not (and how intense/accurate), anywhere from maybe 15 to 38 tiles between the edge of the towers is recommended.  Crossbows actually have their range ''reduced'' by extra height in DF, so all you need is 1 level up to keep enemy archers from using your fortifications against you, and you're set.  (Channeling a defensive moat further out will also work, moving potential enemy archers even further away, but also moving non-missile targets that far as well.)

  Side                Below         Ground       Archer       Roof
  view:               ground:       Level:       Level:       Level:
                       ╔═╗          ╔═══╗        ╬╬╬╬╬        ·····
     ___               ║<║          ║X..║        ╬>++╬        ·+++·
    ╬>__╬              ║.║          ║...║        ╬+++╬        ·+++·
 ___║X__║___           ║.║          ║...║        ╬+++╬        ·+++·
     <......           ║.║          ╚═══╝        ╬╬╬╬╬        ·····
   (Fortress->)

This can hold 3 archers/side, and has the potential to be as many "archer levels" tall as you wish.  A top-level "down stair" is necessary to build the "roof" - might as well build an up/down stair instead, no real reason not to.  Remember to use the "corners first" technique when necessary. (See [[Tower]].)

All told, for a simple 1 archer-level tower, this takes just over 50 stones or blocks (plus 25/extra archer level).

Larger towers (or this with larger floors on higher levels) could house barracks, practice ranges, and other facilities.  Just expand to preferred size with floors, and then attach walls to those to act as a base for the next level of building.  Add more stairs (adjacent to each other is always better) if high traffic is anticipated.

====Siege engine turrets====
If it's big enough, build a [[siege engine]] inside a pillbox. Since siege engines cannot fire at targets higher or lower than them, the device needs to be on the same [[z-level]] as any targets, but this could be across a large gap to a nearby plateau. Only a single tile of fortifications is needed to fire through the wall.  Position the tower to fire where invaders tend to congregate.

You will want to guarantee that enemies do not approach the position and scare the civilian operators - this distance has been reported to be up to 20 tiles or so.  Dig a moat, have some intervening valley or build some secondary fortifications to keep enemies at a distance. Unlike walls, fortifications on the same z-level do not block siege engine missiles, at any range.  Unfortunately, if an enemy can walk up to them, fortifications will protect enemies from your archery fire (but not siege engine fire.)

===Control Room===
Have one (large?) room (or several stacked on top of each other) for all defense-related levers, and central to idle dwarves - near your [[Activity zone#Meeting Area|meeting area]]s and [[noble]]s quarters, with one or more halls or stairs leading to it for quick access. Connect a lever to all those doors and hatches as the first lever to be pulled in an emergency, and the respondent will lock themselves in for you, guaranteeing that they will then have nothing else to do but stay there and pull levers.

It may also be an idea to have a second lever to at least one door, for emergency access.  And possibly to add a stockpile of booze and food or a well for longer sieges.

===AI abuse===
Taking advantage of the game's Artificial Intelligence and [[path]]finding  is a whole article in itself.  Try leaving a door un-forbidden during an attack.  When the bad guys approach the door, forbid it, and the enemy will wander off.  Unlock it again, and they turn around and head back towards the door again.  You can get enemies to march back and forth over a set of traps this way, or lure them deep into a complex trap. This could be fully automated via [[pressure plate]]s if you're feeling adventurous.

===Bait animal===
[[Restraint|Restraining]] or [[Pasture|pasturing]] a sacrificial animal just outside your walls, but within range of your marksdwarves and/or siege engines, can lure an enemy into attacking that while you cut them down.  Make sure to place a pattern of some walls (or statues, see below) so enemy archers cannot simply shoot the creature from a safe distance.

=====Fortifications=====
Adding a ring of fortifications to help defend the animal against missile fire will keep melee troops away, but invite archers to come adjacent to the fortifications - and under your walls and crossbows.  If you allow any path, the melee troops will try to follow it to the animal - be creative with that fact. 

=====Traps=====
Surround the animal with traps to kill or capture approaching goblins.

Build a long, narrow, and twisty passage, accessible from the outside, possibly even unconnected to your fortress. Build as many simple traps as you like. Place a bait animal inside. Enemy attackers walk right in, and get torn apart by the traps. If any manage to make it to the end, and kill the useless animal, they're surrounded by traps, and no closer to your fortress. 

If the animal is underground, you can build a tunnel above it, channel down, and mark the channel a [[Activity_zone|pit/pond]]. That way, you can "reload" a new bait animal from the safety of your fortress.  Note that this requires using a non-pet-passable door and that falls more than a couple z-levels may injure your bait.

=====Distractions=====
Releasing a [[cage]] full of surplus animals will keep the enemy archers very busy. They may even be out of ammo when your wrestlers show up.  This can be useful as an emergency measure since the animals need to be stored somewhere anyway.

=====Vanishing act=====
Having a linked drawbridge that can open/shut (perhaps on both a lever to open and a nearby pressure plate to close), to lure the enemy in under your guns and then protect the animal when they get too close (for multiple uses.)

=====Vs. building destroyers=====
For [[building destroyer|building destroyers]], spare statues can serve the same purpose as bait animals.  Building destroyers will hunt down and destroy structures, so carefully placing them can control its movements.

=====Trap chokepoints=====
Enemies can be herded by constructed features. If you have a particular zone covered by catapults and would like enemies to pass through it, strategically placed walls can make enemy pathfinding more favorable. A trap occupying a single tile in the middle of a barren plain is likely to never get triggered. However, if walls are placed in a cross-hair pattern around the trap, animals and invaders are much more likely to pass over it as they wander across the map. This can be a very useful trick when capturing wildlife with cage traps. 

<pre>
+++++║+++++
+++++║+++++
+++^^║^^+++
+++^^║^^+++
═════^═════
+++^^║^^+++
+++^^║^^+++
+++++║+++++
+++++║+++++
</pre>

=====Pathing slowdowns=====
If you're playing on a low-powered machine and you close up all entrances to your fortress during a siege, your game may grind to a halt and/or crash as the siegers continuously fail at pathfinding into your fortress. Bait animals may alleviate this.

==Entrance designs==

===Airlock defenses/buffer zone===
Build two walls, each with a drawbridge. Build the trade depot in the buffer zone between them. Keep the outer bridge open, and the inner one closed. When the merchants appear, put crossbows on the walls to guard their approach. Once all the merchants are safely inside, close the outer bridge. Once there's no enemies left in the buffer zone, open the inner bridge so your civilians can start loading up the depot. 

The airlock pattern can be useful even without putting the depot there. Let a few siegers in at a time, and crush them. Reset the traps, Rest up the soldiers, and repeat.

===Siege Engines===
One effective way to have [[siege engine]]s (help) defend your fortress is:

'''One ballista vs 3-wide hallway'''
 ══════════════════════════╦═════
 Entrance++++++++++++▼·····║▐▀\
 Entrance++++++++++++▼·····╬◄═«
 Entrance++++++++++++▼·····║▐▄/
 ══════════════════════════╩═════

Using this design you can cripple an army using a well timed volley.  The hallway can be much longer than shown if you wish, as ballistae have extended ranges well over 100 tiles.  The channeled area is necessary, as civilians (siege operators are "civilians") will run when enemies get within about 5-10 tiles of them, regardless of the actual path to that threat.

====Ballista battery====
Three (or more!) ballistae can be put into a "battery" if overlapped - one per tile-width of the hallway, with each ballista aiming down their row of tiles.  

                                 ╔═══
 ══════════════════════════╦══╦══╝▐▀\
 Entrance++++++++++++▼·····╬  ╬▐▀\◄═«  (~ammo~)
 Entrance++++++++++++▼·····╬▐▀\◄═«▐▄/
 Entrance++++++++++++▼·····╬◄═«▐▄/ (~ammo~)
 ══════════════════════════╣▐▄/ (~ammo~)
                           ╚═════════

Be sure to use fortifications to prevent dwarves from wandering in front of the ballista to their deaths. If desired (and you have the <s>man</s>dwarfpower to spare), catapults may be put behind those, as they shoot safely ''over'' workers in front of them.  Although less effective than ballistae, it's a little more firepower - and that can't be a bad thing.

For added flavour, channel out one or more tiles down the length of the 3-wide hallway and install retractable bridges.  When invaders attack, retract the bridges, forcing them into paths that are only 1-tile wide.

Adding additional channels on either side of the hall will allow stray ammo to be recovered at a later time.  Make sure to add locked doors, to prevent siege operators from walking down below enemy archers during a battle.

===Flooded entrance===
Using a chamber as your entrance alongside a chamber full of water and some machinery you can flood or drain the entrance at will.

The basic premise requires two levers, two [[screw pump]]s and two [[Gear assembly|gear assemblies]]. The amount of power required and the number of additional components needed to get the power to the screw pumps varies depending on distance/setup. One pump is placed to draw from chamber 1 and dump into chamber 2. The other is set in reverse. A gear assembly is placed next to each pump and connected to the main power system. Each gear is linked to a lever. Now at the flip of a switch you can submerge your entrance with [[water]] or [[magma]] for easy, secure defense against creatures that aren't amphibious or magma-dwelling, depending.

[[Image:Entflood.jpg|200px]]

The picture above shows the design in action. The green pump is currently on while the red has been disconnected through the grey marked axle. The yellow X is just to mark that there is a channel under the axle.

=== The "Reverse Battlement" design ===

Level Z+0 (ground):
   ...............
 F ═══════════════
 O .≥.g≥...g......
 R ..≤......g.....
 T ...g≤..g....... <-- enemies enter here
 R ..≥......g.....
 E .g.≤.........g.
 S ═══════════════
 S ...............

Level Z+1 (bridge):

 E ·····║+++║·····
 N +++++║+++║+++++
 T ·····╬☺++║·····
 R ·····╬☺++║·····
 A ·····╬☺++║····· <-- archers shoot them from up above
 N ·····╬☺++║·····
 C ·····╬☺++║·····
 E +++++║+++║+++++
   ·····║+++║·····

Note that in this diagram, the fortress interior is to the West, and the enemy forces come from the East. The marksdwarves on the bridge with the [[fortification]]s are one level above the [[goblin]]s (or other attackers), who will pass under the bridge and charge on toward the west. As the first clear from under the bridge, they are targeted from behind (which is one level above), as the marksdwarves wait in ambush. This allows the marksdwarves to face far fewer enemies at any one time, at least to begin with, and any enemy archers must clear the bridge, take their lumps, and then return fire back the other way before the marksdwarves are ever under attack.

If you're feeling especially nasty, make the tunnel really long into the mountain and add a ballista battery (see above).

''(Adding ammo stockpiles, of your best quality bolts, to these stations will speed up reloading for longer sieges/battles.  Even adding small, convenient food and alcohol stockpiles is not unheard of.  Some designers place access to/from archery ranges very close to these stations, for faster deployment.)''

===Twisty maze===
A maze of turns and blindspots patrolled by quality military can be a very formidable defense.  Wide enough for wagons to pass though, but with no clear shots for any ranged weapons.  Missile weapons do have a minimum range, so if a target is closer than that range, they will instead just charge to melee - and meet a dwarf with a much better melee skill. Downside to this is that you'd be mixing it up in melee all the time, but so long as you have at least 10 dwarves greeting the goblins as one coherent mass, you should win.

Variations on the twisty maze include:

*A wagon-wide twisty maze, and a not-so-twisty 1-tile wide hall o'traps, with a drawbridge that can force one or the other as the only [[path]] into your fortress.

*Making the side of the maze into fortifications, with a channel separating the fortifications from the actual floor of the maze, and having your archery targets on the other side of the fortifications so your marksdwarves can practice.  When the goblins round the corner, they charge through a hail of crossbow bolts, and drop dead.


===Forced Detour===

This particular design works well with plenty of archers, siege engines, and other ranged weaponry.  A walled maze gives melee an advantage, but an open maze gives advantage to ranged attackers.

 ++++++++++++++++++ENTRANCE+
 ══╦════════════════O╞═╡O╦══  <-- Bridge 1
 +☺╬·+++++++++++++++++++·╬☺+
 ++╬·+···············╞═╡·╬++  <-- Bridge 2
 ++╬·+·+++·+++·+++·+++++·╬++
 +☺╬·+·+·+·+·+·+·+·+·+++·╬☺+
 ++╬·+·+·+·+·+·+·+·+·+++·╬++
 ++╬·+·+·+·+·+·+·+·+·+++·╬++
 +☺╬·+++·+++·+++·+++·+++·╬☺+
 ++╬·················+++·╬++
 ++╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╗·+++·╬++
 ++☺++☺++☺++☺++☺++☺╬·+++·╬☺+
 ++++++++++++++++++╬·+++·╬++
 ++++++++++++++++++╬·+++·╬++
 ══════════════╗++☺╬·+++·╬☺+
               ║+++╬·+++·╬++
               ║+++╬·+++·╬++
               ║++☺╬·╞═╡·╬☺+  <-- Bridge 3

The 3 tile wide lane is for traders, so if your [[trade depot]] is located before this set-up, cut it down to a 1 tile lane to slow down invaders more.

===Bridge Use===

[[Image:defense_3bridges.png|thumb|right]]
An example of bridge construction tactics to deal with vile forces of ''any'' size. (See picture).

*'''Bridge 1''' seals off the entire base
*'''Bridge 2''' forces everyone to take the long, winding, heavily trapped/defended path of death.
*'''Bridge 3''' seals the inside of the fortress

Clever triggering of the bridges allows you to break the hostile forces into smaller chunks to be trapped in the courtyard while being caught in traps and a crossfire of arrows from the fortifications around.

===Pulling it all together===

Using ballistae, marksdwarves, bridges, traps, and guard animals in tandem brings up a few more considerations.

*Given the tendency of ballistae to pierce through enemies and knock out an entire row, forcing invaders along narrow paths can score multiple hits per shot.  This is the most efficient use of these valuable pieces.
*Be careful about positioning your marksdwarves and ballistae opposite each other as an unlucky bolt might pierce a battlement on the other side.
*Ballista are operated by Dwarf civilians. Therefore putting your ballista right next to the path the [[v0.31:Goblin_christmas|friendly neighbors]] use, will cause your BallistaDwarfs to run off right as you want them to fire.
*Adding a stair back at the entrance of a trap hallway allows dwarves to access loot that falls; it also forces anything that dodges off the edge to walk the entire length again.
*Filling the walkway with traps is also highly recommended.  Throwing random weapon traps around the map in high-traffic areas isn't a bad idea.  Traps are cheap and effective, use them liberally.
*War animals are convenient for catching thieves.  They also serve as bait.  Don't place them where enemy archers can reach them easily, and keep them well out of range of the inevitable hailstorm of bolts.
*Bridges, and for non-building-destroyers, doors, can be used to control the movement of enemies.
*Don't use cage traps as the front line of defense.  Currently, a goblin squad will follow its leader indefinitely, and if its leader is caught in a cage the rest of the goblins will just stand there until they find [[Wood cutter|something]] [[fisherdwarf|to]] [[hunter|do]] rather than exploring the lovely, pointy playground you've created.


                        Base
                        ║╞═╡║
                        ║D+D║ ╞═╡= Bridge
                        ║D+D║  D = War Dog (chained)
   ☺  ☺  ☺  ☺  ☺  ☺  ☺  ║+++║  ^ = Trap
 ╔╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬═╝+++╠════
 ║.......................+++║▐▀\
 ║.++^+^+^+^+^+^+^+^++++++++╬◄═«
 ║.+.....................+++║▐▄/
 ║.+.....................+++║▐▀\
 ║.++^+^+^+^+^+^+^+^+++..+++╬◄═«
 ║....................+..+++║▐▄/
 ║....................+..+++║▐▀\
 ║.++^+^+^+^+^+^+^+^+++..+++╬◄═«
 ║.+.....................+++║▐▄/
 ║.+.....................╞═╡║▐▀\
 ║.++^+^+^+^+^+^+^+^++++++++╬◄═«
 ║.....................+++++║▐▄/
 ║....................>+++++╠════
 ╚════════════════════╗+++++║
                      ║╞═══╡║
                     Entrance

                                         Base
                                        ║╞═╡║
                                        ║D+D║ ╞═╡= Bridge
                                        ║D+D║  D = War Dog (chained)
                   ☺  ☺  ☺  ☺  ☺  ☺  ☺  ║+++║  ^ = Trap  c= cage trap
                 ╔╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╝+++╚═══╗ 
 /▀▌╔════════════╝.......................+++.+..║ P= Gathering Pit
 »=►╬++++++++++++..ccc+^+^+^+^+^+^+^++++++++.+..║    connected to
 \▄▌║++++++++++++..+.....................+++.+.P║    Base next 
 /▀▌║++++++++++++..+.....................+++.+..║    z-level down
 »=►╬++++++++++++..ccc+^+^+^+^+^+^+^+++..+++.+..║
 \▄▌║++++++++++++.....................+..+++.+..║
 /▀▌║++++++++++++.....................+..+++.+..║
 »=►╬++++++++++++..ccc+^+^+^+^+^+^+^+++..+++.+..║
 \▄▌║++++++++++++..+.....................+++.+..║
 /▀▌║++++++++++++..+.....................╞═╡.+..║
 »=►╬++++++++++++..ccc+^+^+^+^+^+^+^++++++++.+..║
 \▄▌╚════════════╗.....................+++++.+..║
                 ║....................>+++++.+..║
                 ╚════════════════════╗+++++.╔══╝
                                      ║╞═══╡.║
                                      Entrance

 {{Military}}
{{category|Fortress defense}}
{{category|Design}}