a great strategy on citys with walls is get the rein forcment spell and use that whaenever you can but dont attack while youre waiting for the reinforcment spell to recharge use the lightning spell on the enimies in side the walls ive one a city that had 42 guys in it and i didnt even us reinforcments
you should be able to stop yoyre units so you can build a big army and them just destroy the enemy or somthing like what the gual do in the first level with all of their troops