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->Traps/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 only be built indoors.
Stone-fall, weapon and cage traps will be triggered by (most) any hostile entity entering their tile, with the exception of kobolds, gremlinsVerify, and various demons. Additionally, any unconscious creature, including your own dwarves, pets and war dogs, will also set off such a trap.
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, so locking the trap-filled area may be necessary to prevent your mechanics from marching to their horrible deaths.
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, magma men 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. As 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.
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. 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 chance that the trap will become jammed with a corpse each time it kills a creature. Any nearby adult civilian dwarf (including nobles) will automatically clean a jammed trap.
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".)
Weapon traps benefit from being constructed with high quality mechanisms, striking with higher accuracy 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 out of your dwarf's hands, 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.
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.
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, you'll want to remove the thief from the cage and kill it, though it may also be possible to mark the bag for chasming from the Stocks screenVerify.
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. 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.
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.
You can create even more elaborate traps with imaginative use of pressure plates, levers, 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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