The AI did this to me the other day playing as Danes (although it didn't ruin my campaign). Did you try loading and replaying a few turns? Don't know if it would work but I like to save every 3-4 turns in case something really stupid happens. In any case, this is the ONLY purpose of the battle timer. Stupid idea really ("Ok men, we've taken the walls, fought through the streets, lost half our forces and there are only 4 militia in the city center, but we've been fighting for 45 minutes and i'm getting hungry so pack it up boys, we're going home" [This happened to me once in RTW, but the remaining enemy died about 3 seconds after the timer hit 0:00 and it gave me the win])