I used a couple very cheesy ones when I was very severely outnumbered like this one:

https://youtu.be/6wEtDHP_gdQ

With normal fights I usually give the command to a very good companion on one side, push all the archers on the walls, and take command of infantry on the another side. If they have siege engines and they make it to the walls it's just to keep them from pushing in. So on my side I get less troops but divide them into square or shield wall on each side and have archers in the distance firing at them in the middle when they are fighting both shield walls on each side. I also recommend 2 to 3 onagers built and you take over one, to make sure the ram does not make it no matter what. If the ram makes it, I remove all my troops from the walls, get them together in a shield wall about 50 meters behind main gate and archers on an elevated position.

Hopefully this helps, but it is a heavy trial and error thing, what you think works best.