This is an old thread now, but I thought of a good way to keep invaders from getting up close to the fortifications. Run some water filled channels outside the fortified wall. That way enemies that cant swim cant get right next to them, while your troops can freely enjoy some live target practice.
I'm on my third map, and just found out about the sieges. My defenses never took that into the planning. And I have learned some crucial details about living space, and about how far oversizing rooms I have been doing. So I'm planning to start a fourth map, and this time laying things out with both these things in mind.
I'm thinking of mining off the first couple columns of rock from a long stretch of the cliff face, then indenting the 30 spaces north and south of the center by a few more spaces, and then making a 20 by 20 indent in the center of that. Maybe a bit smaller, I'm not sure what the archery range limit is, I never paid attention to find out. Anyway, then build fortifications into the out-facing walls, and the north-south facing walls. Put channels outside those, and then a line of traps on the other side of the channels. Normally you cant build traps outside, but if you indent the cliff some, then you are "inside the mountain", even tho its still outside your fortress. Just getting close to the indent should be hazardous to an invader's life. But once in it, you can be fired at from TWO directions, instead of just one. Then have drawbridges at the back of the indent, to pancake anything getting that close. That should take care of most problems. But Then add a second channel running next to the water channels, for a steam defense, for the really bad invasions.
Dont know if my skills are up to all of that, but it should be fun trying! 