since build 33 there's a new feature that gives mappers the ability to assign a trigger_multiple the key "teamnumber". Well, in my map university (siege-mode!!!) i used this, to make it impossible for the defenders to open the big doors. The problem is, that the teamnumber doesn't switch like the spawns do, so the purple team can always use the doors and the the blue time never.
I want to say that this feature should be adjusted to the SIEGE gamemode.
(the used mapfile in the demo is beta 8!)