Traps are a reliable and cost-effective method for defending any fortress. Unlike soldiers, they're always on duty, and don't need to be carefully managed. On the other hand, they are immobile and can only lie in wait for foes to walk over them. To build a trap, go to the uild-> raps/Levers menu. You'll generally need one mechanism, a dwarf with the mechanic labor designated (ranks in this skill reduce the time to place a trap), and at least one other component depending on the type of trap - a stone, a cage, or one or more weapons. They can be built indoors or outdoors, and require a level ground square with no other constructions in them.
Stone-fall, weapon and cage traps will be triggered by (most) any hostile entity entering their tile, with the exception of kobold thieves. Additionally, any unconscious creature, including your own dwarves, pets and war dogs, will also set off such a trap. (Kobold thieves have the ability to avoid traps, though it seems to have a small amount of randomness built in.)
Note that, in combat situations, Mechanics (and others) have a nasty habit of wanting to reload (or clean) traps when they are triggered, regardless of who or what might be out there as well. Forbidding traps after they are built will keep Urist McSuicide from deciding to reload a stone trap in the middle of a siege. Just remember to unforbid them when things calm down, so the traps are all ready for next time.
 Stone-fall Trap
The simplest trap to construct, a stone-fall trap is essentially a stone suspended up in the air which is dropped on intruders when the trap is triggered. These are a popular defensive measure early on, as the components needed are readily available as soon as you start mining. A single stone trap will kill or severely maim most humanoid enemies although trolls, magmamen and hardier creatures may take two or three to drop. Mythical creatures such as dragons, hydras and titans will take upwards of five or six. After being used they need to be reloaded with another stone by any dwarf with mechanic skill enabled, a task which your dwarves will see to automatically. Being that stonefall traps do not alert you of ambushes when triggered by hidden invaders, this can frequently lead your mechanics into peril.
Contrary to expectations, mechanism quality does play a role in the effectiveness of stone-fall traps, though the extent of this role is not fully known - at the very least, high-quality mechanisms may be able to fling their stones at targets with greater amounts of force than low-quality mechanisms.
- Components used: mechanism and an ordinary stone
^= no stone loaded
 Weapon Trap
Weapon traps consist of any number of deadly instruments rigged to a mechanism. When an intruder sets off the trap, the weapons spring out and strike the poor sap. You can add up to 10 weapons to a weapon trap, and they will all attack together when set off. This gives the potential for dealing significant amounts of damage at once. There are also special giant weapons such as the large, serrated disc that are specially designed for use in weapon traps. Unlike stone-fall traps, weapon traps automatically reset after being triggered, ready to splatter the intruder's friends. But there is a 50% chance that the trap will become jammed with a corpse each time it kills a creature. A nearby dwarf will automatically clean a jammed trap; this does not require the cleaning labor or the mechanics labor.
If there are weapons that require ammunition (crossbows, bows, etc) in the trap, each such weapon requires appropriate ammo (arrows for bows, darts for blowguns, etc), and will have to be reloaded occasionally. Weapon traps using only ranged weapons will not require cleaning, but once all the ammo runs out, that weapon is done until reloaded. If a ranged weapon runs out of ammo, other trap weapons will still trigger. If non-ranged weapons are included in the same trap and jam, the entire trap jams until cleared. Ranged weapons in traps only target the creature triggering the trap, just as any other weapon - if the creature steps onto that trap, the trap targets that creature, once per triggering, until all ammo is gone. (This means "Range = 0".)
Unlike other traps, weapon traps benefit from being constructed with high quality mechanisms. Weapon traps are more accurate the better their mechanism. Since weapon traps can be constructed with multiple weapons, and each weapon's attack is calculated separately, traps with multiple weapons benefit more from high-quality mechanisms than do traps with only one weapon. If you manage to get an artifact mechanism, stuff it with as many weapons as possible!
Weapon traps are a nice way of getting rid of any cheap, mediocre captured weapons, wooden weapons you don't need for sparring and weapons your dwarves can't use. Due to the bundling of weapons you don't have to worry much about the minor damage they would cause separately. There is no difference between low quality crossbows (wood, bone, copper) and high quality steel and adamantine crossbows—the metal only changes their effectiveness in melee. Thus wooden crossbows at the bowyer make for easy-to-produce trap weapons.
- Components used: mechanism and whatever weapons you want, limit 10.
^= jammed or out of ammo
 Cage Trap
Cage traps capture creatures that set them off in cages. After a creature is captured, it's stored, cage and all, in an animal stockpile. Then the trap is reloaded with another cage. You can do all sorts of fun things with captured creatures. Creatures in cages will not be fed, they will survive indefinitely without nourishment. Cage traps will alert you to ambushes when triggered by hidden invaders, making a useful forward defense mechanism.
It is possible that dwarves bring water to cages, but that means that you have someone friendly also locked in the cage - like a dwarf kid snatched by a goblin babysnatcher. In this case remove the poor fellow using the goblin's inventory screen. A cage trap is one of the most effective ways to defeat powerful beasts, as even a glass cage (aquarium/terrarium) can imprison a bronze colossus. This was expected to change. Also, the creature captured in the cage will have no effect on the cage itself (a caged fire imp will not burn down a wooden cage, for example).
Cage traps are also useful for catching animals for use in the meat industry.
 Berserk dwarves
Cage traps will capture berserk dwarves, so it might be wise to keep them stored instead of killing them (for happiness purposes), since relationships they have with other dwarves are maintained. However, unlike other caged creatures, dwarves will starve to death over time inside a cage.
 Upright Spear/Spike
These traps can only be utilized by attaching them to levers or pressure plates, which when triggered will cause the spikes to extend from the ground and, when triggered again, to retract back into the ground. They don't slow or hurt any creature walking through them if they are not currently triggered by a lever/plate, neither when up nor when retracted.
The lever task can be set to repeat which makes the spikes go up and down, but there is a high chance a creature will pass the spike trap before or after they shoot up, so you may want to build a row of at least 5 traps.
The damage done depends, like with weapon traps, on the number and quality of the spears and spikes used. One trap can be loaded with up to 10 spikes or spears. The fact that they do piercing damage makes them useful against more powerful foes which are most easily killed by damaging their organs. Using a single, no-quality spike made from low-damage material can be useful in attempting to cripple nobles when killing them outright is not desired, but much luck is in involved in the outcome.
Spikes will, when triggered, damage creatures unaffected by other traps (kobold thieves and demons, but also dwarves, pets and allied creatures). The traps' friendly fire means they make effective anti-immigrant traps. There are also reports that the spikes are effective against demons, although spikes which are not made of steel may melt.
Contrary to what one might think, the spikes appear to do no (additional) damage if a creature falls on them. There is therefore no advantage to putting the spikes at the bottom of a pit trap and you still have to connect them to a lever or pressure plate to cause injury -- not that you shouldn't do that anyway.
Stuff does get stuck in spikes when they are triggered. Any corpses will appear as an item inside a spike's 'building' when they are out (the same way mugs and whatnot appear inside workshops when you first make them). Retracting the spikes lets the corpse out. If a corpse is in a square when the trap is triggered, it will be skewered again on the spike.
Testing has shown that there is no obvious limit to the number of creatures a single spike can strike and impale-testing with recruits ordered to stand on a retracted spike resulted in all six recruits getting killed instantaneously and stuck on the spike. Use with extreme caution.
- Components used: 1-10 spears or spikes, plus further mechanisms for linking to triggers.
 Other Traps
You can create even more elaborate traps with imaginative use of pits, pressure plates, levers, grates, supports, water, and/or magma, creating sacrificial altars (blood for the Blood God!) and whatever else you can think of. Watching those goblins try to find a way out of your drowning chamber as it begins to fill is really quite satisfying. These are best made in a large, repeatable mass killing, way. If you make a trap that kills 10 or so goblins, that only works once and you have to rebuild it, wasting time you don't have during a siege, you're not trying hard enough.
See Trap design
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