Open field tactics with all-cav army (especially if you have horse archers):
Order everyone to follow you and ride at the enemy. At the last second before you hit, order knights, mamlukes and/or lancers to charge, then you peel away (to the right, preferably). If you have horse archers, have them continue following you. The enemy will turn to pursue you and take the full brunt of the charge on their unprotected sword arms. Once the charge has thrown the enemy into disarray, have any horse archers charge and they'll skirmish and pick off stragglers.
This works best in desert, steppes, and plains. Not so much in mountains: there, you're at a severe disadvantage and may want to wait for the enemy to come to you if possible (hopefully, your archers will be picking them off as they approach).
Siege defense is fairly easy: it's usually best not to order your troops to do anything, as they'll automatically be deployed to best advantage, with archers on the walls and infantry at the gap in the walls. You may want to order your infantry to spread out a little if the gap is wide, but other than that, there's not much to do. I generally put 25-50 (depending on the castle/town) top-tier infantry at the top of the garrison list, then pack the rest of the garrison with high-level archers or crossbowmen (about 50), and then Swadian militia, a cheap filler troop that usually spawns with crossbows (unless I'm playing Rhodoks; then I just use their tier 2 crossbowmen). With a good defensive castle like Grunwalder Castle or Uhhun Castle, chances are very few attackers will even make it to the walls to engage in melee, being picked off by arrow/crossbow bolt before they make it up the ladder.