I played Overlord II while I get stuck during rescuing green minion hive. I've killed all the sentiels and run the elevator but the main gate remains closed.
I tried to walk through the castle and search google but I found nothing.
It's a bug - I've replayed it only to have green hive disappear from game (it reached teleport but the green minions have not been unlocked).
PS. If Powerlord give comment as answer I'll accept it instead of this.