The pressure plates will lift the bridge, blocking off the entrance to the fortress, so that incoming goblins will try to use a different entrance.
In order to get to another entrance, they need to cross the hatches, which will drop their equipment (and them) thirteen levels down to the metalsmithing area of the fortress.
This process will hopefully repeat until successful.
Rampartclinch
Submitted by: ToonyMan - 2009-06-16
This, is EMAZING! It looks like you even planned it out before you started! I feat I could never accomplish! Good show. Let's see this gizmo in action now, shall we?
Submitted by: Fire Imp - 2009-06-18
Very impressive. I especially like the symmetry.