The militia commander is the only one capable of any kind of combat at the moment, previous sieges have took a serious toll on the military. That said, the he has taken out two sieges singlehandedly, and another with a partner, so he's no laughing matter...though trolls knocking him around make this siege rather difficult. Also, no access to iron/steel from what I found. The lever at the end of the hallway gets the party started.

Could just hide in the fort until the siege leaves, but where's the FUN in that?