When I was playing Byzantium, Hungary had declared war on me. I caught the fortress of Bran defended only by a general, who suffered a mysterious fatal accident. That left the fortress undefended and I waltzed in and occupied it. I saw a large Hungarian army northeast just on the other side of the mountain range, and worried about it, but for about 10 turns it didn't move. I thought the AI was being passive, and moved most of the troops out of Bran to deal with a rebel army that appeared southwest of there.

Well, it wasn't being passive; it was waiting for another army to be completed. Because it started moving, only not alone. The large army (or what I thought was the large army) swung around the mountains through the pass and besieged Bran. The LARGER army (which I never knew about, teach me to keep spies out in advance of the borders) moved down and besieged Bucharest. I could only relieve one. I relieved Bran and lost Bucharest, but took about 1/3 of the army out defending against the assault. I had to go back and lay siege to Bucharest with an all horse archer army because I hadn't teched Bran up to build infantry, then wait until it starved out to retake the city.