40d:Trap

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 build->Traps/Levers menu. You'll generally need one mechanism, and at least one other component depending on the type of trap. They can be build indoors or outdoors, and require a level ground square with no other constructions in them.

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. After being used they need to be reloaded with another stone, a task which your dwarves will see to automatically.

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Components used: mechanism and an ordinary stone

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 as many weapons as you want to a weapon trap, and they will all attack together when set off. This gives the potential for dealing hilarious amounts of damage at once. There are also special giant weapon parts you can build 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 (20%?)chance that the trap will become jammed each time it is triggered. Dwarves will automatically clean jammed traps. If there are weapons that require ammunition in the trap, they will also have to be reloaded occasionally.

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Components used: mechanism and whatever weapons you want.

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 appropriate animal stockpile. Then the trap is reloaded with another cage. You can do all sorts of fun things with captured creatures.

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Components used: mechanism and a cage.

Upright Spear/Spike

These are not active traps, but a hazard you can place in order to impale trespassers. You can combine these with retracting bridges to create a spike-filled pit trap. You may need to clean the corpses off the pointy bits occasionally.

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Components used: Spears or spikes.

Other Traps

You can create even more elaborate traps with imaginative use of pits, pressure plates, levers, grates, supports, water, magma, 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.