So here's the situation: I'm attacking Cobra's Castle (Citadel built) with 4 birds. There are tier 1 units there. I fly forward and strike, then all turns are skipped except for the catapult (even the towers). It won't let me take any actions. We reloaded several times and tried different approaches to the battle — same thing every time.
If you attacked and can't win, then according to the rules you must retreat. :D
Cobra, beer's on you! :p