I came up with a design shortly after the z-level release came out, and although I haven't gotten a chance to actually test it out yet, it should still work.
It is, essentially, a pit. But it is a pit that allows for less digging, while still keeping a respectable number of kills. It can plunk goblins into a space where their goodies can be easily and safely claimed, and maybe even finished off with a few upright spikes if they survived the fall. Or, if you don't want the extra clutter, flood the bottom of the pit with magma.
So here's how it works: You channel two-tile (or whatever you want) wide pit for a very long distance in front of your fort. You decide the depth. Then, build retracting bridges over the top of it all. Finally, build the largest drawbridges you can along the side, and set them to raise inwards. Connect all bridges (retracting and flinging) to one bridge. When a siege comes, pull that lever at an appropriate moment.
This will allow you (provided the drawbridge doesn't just fling them in a random direction) to extend the reach of a pit by ten tiles to either side, and if you build a long enough walkway you can collect a rather nice number of gobblies. They get funneled into the central pit, drop down a fair number of levels, and then do their jelly impression when they hit the bottom.
Or, go with something along these lines:
code:
(extends for some distance)
#...#
#...#
#...#
#...#
#...#
#...#
#...#
#...#
#.....#
#.......#
#.........#
#...........#
#...........#
#...........#
#...##╬##...#
#...#▒╥▒#...#
#...#▒║▒#...#
#...#▒O▒#...#
#...#.X.#...#
#...#####...#
I used a catapult in this diagram because they're more fun to look at.
This allows for one entrance for both caravans and sieges, since it's wide enough. The 'X' is an up/down staircase, leading to an ammo reserve below and a marksdwarf station (if you feel like carving out the second level enough for your shooters to hit something) above. A good siege weapon should be able to handle the larger amounts of enemies that can come through the corridor, and the wider space allows for a greater chance of the projectile actually making it far enough to do something.
Note that if you are going to use catapults, in may be more beneficial to use a design that puts more of them in, such as this:
code:
###╬╬╬###
#.▒╥▒...#
#.▒║▒...#
#.▒O▒...#
#•••▒╥▒.#
#X••▒║▒.#
#•••▒O▒.#
#..▒╥▒..#
#..▒║▒..#
#..▒O▒..#
This allows for both an auxiliary stone supply, and more catapults flinging stone. I would not recommend ballistae in this setup, as bolts have a funny habit of killing stuff.
But then again, all my designs are theoretical. I have never actually had a need for a defense system, and as such I have had no chance to test any for effectiveness.
[ March 22, 2008: Message edited by: Kagus ]