Sounds a lot like Sieging in Aion, which is one of the few PvP related activities I liked in it.

Basically if you were attacking you'd have to break down the gate, kill all of the players, go down to the basement and kill the Boss which was a giant NPC. It was kind of a mix of PvP and PvE. There were siege weapons and the like there as well.

He's not saying that all the players would do is craft siege weapons but that they could be used to help defend. During the actual defense players would have to participate in the combat.