In Dwarf Fortress can create a lot of different traps, but even so, some do not. For example:
1. Rolling stones or logs.
Let's say, you can set a trap Rolling on the mountain. When activated, the right arm, they start to slide in the direction of the adjacent tile, which has a slope. And the cattle of the slope, can move on a flat surface is 5-2 tiles depending on the soil, obstacles, and how long the fall.
2. Hot tar.
Traps over the corridor (from the bottom will take enemies) makes a huge cauldron with burning pitch.
(+) - The enemies are hit by severe burning resin.
(+) - All the things the enemy melts unfit for use.
(-) - To maintain the temperature of hot tar. Therefore rekomekduetsya use only in the defense of the siege.
3. Burning. Not trap, but ...
Burn straw, wood, dry grass etc. with flint or burning arrows. This is sorely missed, for an overall strategy. Fire is cool.
4. Container with small stones for the defense of the siege.
Razdrodit stone into smaller pieces and then put them in the right container, and this container is set on the ramparts. And then, during the siege, the dwarves could throw off emp stones at those who would naoditsya under the wall.
I think a lot of things you can think of. Develop the idea. Add to the list of defensive traps