If you can, choose a group that can research Bombard towers. If you can do that, destroying towns will be REALLY easy. All you have to do is have a high villager count [with a couple of knights and/or infantry], and send them into the enemy fortress and build as many bombard towers as you can in the town and in a matter of minutes... everything will be destroyed.

This also works well for fortifying your own fortress. Make a giant wall of Bombard Towers and no enemy unit will get past them.

BOMBARDS ARE AMAZING! :)