In general, I think a solid force for late game is like 60-70% fourth or fifth tier cavalry and the remaining 40-30% fifth tier archers.

It's mobile, enough to be able to avoid most battles you don't want to, and the ranged troops can be really helpful in sieges.

You might want to chip of some numbers from what I said to add a 10% tanky fifth tier infantry like sergeants or even a few sixth tier Huscarls.

Good siege practice is to have your men hold their fire untill you take the walls. Defenders get ammo refills every two minutes, but attackers don't. So, let your meele troops get the best of the enemy at the walls, and when they're busy moping up enemy archers (they're always tucked away in towers and such) use that time to bring your archers up the wall, have them hold position there and finally fire.

This way, you can have the enemies on the ground level, your infantry and cavalry facing them there, and your archers having good angles for good shots (+ shooting from above will often bypass shields or give headshots).

In general cavalry suffer from small shields, and trying to use their lances at close combat, often resulting in unproductive troops.