That's a bonus that they'll assault a full stack city, especially if it's a fortress/ citadel. The trick is to fort them out the first wave is tough to outmaneuver as they don't have siege equipment, but after that you can out manuever them due to them being slow. Some territories are too expensive/vast to block by forts. If you make a successful fortification, the mongols will be forced to take the long route and will harass someone elses territory. Also I made many cannon forts, and if I could single out a stack I'd attack with a full stack with the cannons being under AI control so they could be bombarded before I attacked. The hordes like to stick togeth wich makes them slow but much easier to manipulate. They do refuse to pay ransom while they arte strong but once you weaken them they can become a cash cow. Caught a king an heir and a general and made 30'000 florins off of them