Try building all round towers and thick walls with crenelations. Enclose your keep by making a square or circular castle. Do not connect your walls to the river nearby. This will make it a tougher castle to manage. If stray enemies break through 100 yards away it may be harder to send troops before farms are destroyed. For example, build Saladin The Wise's castle. You must watch him once or twice before you can get it on your own. It must be exactly like his to work. This castle allows for huge amounts of gold income as long as there are iron mines, stone, and farmland nearby. This setup uses strictly Arabic units though, which is a good thing. Send Horse Archers to patrol in front of enemy castles. They will wipe out plenty of enemy archers and mounted siege equipment. You can then use a siege tower and put your ground archers in their towers and use ballistas to kill the remaining soldiers in the keep. Assassins will then finish off the lord. All this depends on steady cover from horse archers. Try catching enemy castles on fire as much as possible.