I would very much like to have a better death trap for an entrance, something more "Bridge of Khazad-Dûm"-ish. On the far west side of my map I have an area mined out that I'm prepping for a new entrance, but I'm wondering how effective that'll be. What I really wanted to do was have a setup where a narrow retracting bridge would cover a deep pit, and have that triggered by pressure plates so that if an enemy came through they'd be on the bridge when it collapsed.They'd then fall into the pit, and if they survived (unlikely) they'd have to walk back up stairs to the far end and repeat the cycle. However, since megabeasts standing on a bridge will prevent it from operating, that defeats the purpose.
I thought about maybe building several retracting bridges that would hold each other up, connected to the same lever, so even if one failed to retract it'd fall when the others dropped. But that'd mean rebuilding the bridge every time it triggered, which is too much of a pain in the butt.
My current entrance is a 3-wide, 10-long corridor mined with weapon traps--three weapons each, seven at the very end. All around outside I've booby-trapped it with cage traps. After I fought off the fourth siege, capturing about a third of the invading force and slaughtering another third--though they slaughtered a human caravan first--they seem to have lost some will to fight. Most recently the goblins showed up with just six ambushers and about as many thieves, albeit a couple of master thieves. The abundance of caged foes has propelled my interest in a better means of disposing of them.