My very first campaign of M2 was as the Byzantines (I modified the files to unlock them). When the Mongols appeared, they went after Russia, and I had not expanded past Iasi, so I held the bridges against them. When they declared war on me finally, they came en masse, and tried to force a bridge crossing with 3 stacks. I won the long and bloody battle, and when the campaign map loaded back up, the two stacks that were next to the bridge retreated back towards Kiev, while the remnants of the stack that actually attacked me retreated forward. Somehow, it made it across the bridge I was defending, past the reserve stack of troops I had there to help me out, and went nearly a full move into my territory! It took me about 10 turns to wipe them all out, because the two generals kept managing to get away and regenerate their bodyguard units.