"The Wolfenstein Challenge" - "The Trap" [2nd campaign, 2nd mission]

In the final quest of this mission, you need to gather a large army and wait for the empire's troops to attack so you can defend the city. But how long do you have to wait for them to attack? It seems like a very long time has passed... I'm afraid there might be a bug in the game. Although, nothing like this has happened before.