Tower-mounted Mangonels are great siege weapons. In many maps, the keeps start in close proximity to each other. Build your walls out as far as they can go, then place round towers and lookout towers on the wall facing your opponent's castle. Fill them with Archers and Mangonels. In many cases, you can hit the enemy's walls with the Mangonels from you own. Also, if you Archers can reach their walls, you can clear his Archers from your own walls.