Explosion trap.
Side view
#~#
#c#
#%#
Top view
#%#%#
.^.^.
#%#%#
~ - Magma
c - Magma-proof floor hatch
% - Wooden barrel of booze
. - Floor
# - Wall
^ - Pressure plate, linked to floor hatches
The result is a corridor of explosive booze traps. Also, something fun I made.
Top view
###XXXXX###
...........
-----------
###.....###
###^^^^^##
X - Floodgate
^ - Weapon Trap
- - Raised 1-tile bridge
The hallway should be slightly larger - two Urists wide, about. The floodgate holds back the might of an ocean or some other highly pressurized water source. One end enters your fort, while the other leads outside. Finally, the trap hallway should empty into a large room with a deep pool of water and some kind of drain in it.
Once a siege arrives, station a few military dorfs in the hallway. Siege rushes to meet them and you pull two levers.
The first unleashes the pressurized water, quickly filling the hallway. The second lowers the bridge, flushing everything down past your weapon traps. Your military, who should be trained in swimming, will be unharmed. Enemies will go right over the weapon traps and be grinded to a pulp. Survivors will be dumped into your deep pool at the end, where your swimming military will finish off your drowning enemies.