So if I got it right when you said "the last time you played it" you can re-load the battle to try again?
Well if thats the case here is what I would do:
Have your archers on the walls ofcourse but dont use fire arrows as this slows down their rate of fire. Concentrate their fire at the most densley packed area of enemy soldiers.
Leave any other infantry off the walls but nearby to take any mongols that make it up and over. If you need to you can place 1 or 2 units of infantry on the walls to protect your archers but you want most infantry off the walls so you wont lose them to siege fire.
Also have your spearmen near the walls because you are going to let the mongols breach your gate and possibly walls. Once the breach is made rush some spearmen to defend from the cavalry and infantry that will rush in. Make sure your spearmen are in guard mode.
Now have your cavalry on side streets. As the fighting at the breaches continues your forces may start getting pushed back. If this happens use your reserved cavalry to charge the enemy flanks.
This should bring you victory or a close defeat. Either way rhe enemy army takes heavy losses and you can easily retake the town.
Hope this helps and let me know what happens
