Gotta agree with the previous replies... build at least 2 towers. I sometimes build a couple of ladders as well. Send your hardest unit up the towers (on seperate walls, normally either side of the gatehouse) and try to clear the battlements. Once you've captured the battlements or look like doing it pretty soon, get a unit of archers up there! They can then rain fire down onto the garrison troops below with near impunity.

I've found that the wall towers will keep on firing until you've sent some men down onto the ground but this can be costly. As you're about to take control of the gatehouse have the rest of your force move closer into the gate and make ready. If possible have a unit of Hoplites and cavalry charged, pardon the pun, to storm the gates.

Later on use Elephants to do the same thing, they'll clear the route to the sqaure for you sharpish!

Attacking stone walled cities is one of the best things about the game, possibly excepting trying to find the bloomin square in time! Though I've downloaded the battle timer off/on app to solve this.