Yesterday I was sieged twice, and both times there was an issue with bridges trying to either smash or trap the invaders. Just to be sure: I did link all the bridges to the correct lever, and the bridges that survived still worked afterwards.
First I was sieged by a group of hammer-goblins with their captain riding a Jabberer. I closed off the main entrance and opened a winding corridor completely covered by atom smashers, which were waiting openen. The entire group of about 20 goblins and their captain came in and I hit the switch. The bridges closed and smashed almost all goblins, but to my surprise the bridge that hit the captain fell apart, leaving some stones and a mechanism. But, the casualties were enough to send them running, so I was saved.
But, because the atom smasher somehow failed, I decide to build a drowning chamber in front of the entrance. I set up some pumps next to the giant lake shore I embarked upon, and made a walled of section with two bridges on each end that would trap anything inside when they are up. I'm almost done and the next siege comes: 40 swordgoblins ALL riding Jabberers. That means my atom smasher is useless and I need my drowning chamber finished. When they almost enter my chamber I also notice some trolls, and they are supposed to be building destroyers, so I hope that the chamber floods quickly enough before they wreck the bridge. When the Jabberers run into the chamber I pull the switch and....... Nothing happens! (the switch was pulled) When they are already halfway into my fort I pull the switch again, and one bridge goes up while the other stays down. Then I notice that the bridge that stayed down had a Jabberer standing on it. Weren't bridges supposed to toss things around that were on them when they open? So.... Again my defenses thwarted.
I'm replacing the drawbridges by a different system using retracting drawbridges and gaps so I don't have this again, but I'm puzzled by why all this happened. This doesn't seem to be working as specified, or am I missing something here?