You know what would really fix sieges? if they dident happen EVERY OTHER DAY, seriously what do they want? ambushes i can see but why in the hell are you going to siege a fortress of 100-200 with 30 guys when you know they have defences that can most likely fend off 600 gobbos? Sieges should be renamed raids and the entire purpose of these are to exploit an opening- think ambush but on a bigger scale, 30 invisible gobbos with trap avoid- only when invisible so you need to monitor your entrance or have 30 friggin guys inside your fortress. I really hate balancing that relies on killing an existing feature, the trade depot is already a huge hole in your defenses you shouldent have to deal with more though i do agree walls should be more intensive in design.
When i first started playing i saw my skinny wall and envisioned a brocade wall, a mild obstacle but a determined attacker will break through it, so i doubled them up. Just make stone walls breachable but a stone block wall would be tougher to break then a rough stone wall so now the AI has a way in and you have time to formulate a response. You gotta realize that some people dont want to deal with intensive design and micro management just because someone got bored. If you dont like traps DONT BUILD THEM! i personally love them because until the military is properly coded your fortress cant function properly without them.
The root of the problem is the gobbos in the first place- why are they attacking? an ambush while you are trading very valuable items? that makes sense, loitering around the map killing dwarves and other creatures for revenge of a previously botched attack? makes sense. Deciding your going to attempt to breach a massive fortress with 30 guys repeatedly? that doesnt make sense at all esp if theres a hard defense like a draw bridge over a magma/water moat. There should either be regular ambushes or a massive siege that involves over 60 gobbos with abundant troll support, this should be proportionate to your pop and wealth. Give them an ability to pass over moats and scale walls to a certain Z level- theres no such thing as a 5 Z level ladder, if you designed your walls with a moat right below then no ladders YOU COUNTERED THE LADDERS besiegers dont get to say no fair he planned ahead and built intensive strong defenses!
The most important thing is- gobbos should know when to stop, a huge winding passge way with over 100 weapon traps with marksdwaves covering the approach? ok your obviously not going to breach this SO THEY STOP. You will eventually get fat and docile and maybe you will expand to fast and open another route inside- they should have spys on the map constantly watching so when they see an alternate route they should launch a massive ambush with ample door breaching support. You now have a choice to hold up and have no more sieges or when your bored open an alternate route or just never hold up- never force the player they should always be able to choose.