But really, why not.......
A standard 3x3(x3->10 weapons) trap design in an entry corridor is more than sufficient to destroy most seiges. Like all successful attacks in DF it relies on differing weapon types. With 10 weapons in each trap, you'll dice/impale/club virtually anything. First row is either bronze or steel discs. These are your slicers to start chopping bits off and start the bleeding. Second row is bronze or steel spears (or menacing spikes) to kill off any larger creatures that got through your first row. Some of the bigger buggers can take a lot of chopping up, so often spears work better. Even if you just use them as upright spikes on a lever (it's not half as automatic as weapon traps, but it feels more interactive). The third row is either copper or silver hammers. Smashes down whatever is left and hits heavy armour users (the armless, bleeding, now vomiting or unconcious sobs). Good mechanisms help to avoid jams I think, but once you've reduced a seige to 10% of its original size without lifting a finger, your military really should be able to do the rest.
Dodgers sometimes "pinball" back and forth through all the traps, which is always a laugh. If you see some enemies without "trap-avoid" or "building-destroyer" tags then it can be funny to have a wooden upright-spear trap-section attached to a lever with some doors built on either side of the section. Just close/lock/forbid the doors once the enemies are in the section and start pulling the lever. Watch goblin pinball. Watch goblin become superman. Watch goblin slowly die. It's a nice way of using some upright spears that are a bit better than no-quality and gives you some really good options for arena fights later. I don't use training rooms for my dwarves, it's an exploit. Goblins are fine though.
Putting cage traps in a row or two before or after your killing ground can get you prisoners if you want, but aren't really necessary. Works great for your super-gobbo-training-deathcamp though.