Just go the cheap approach, no siege towers at all, position your infantry by the gate outside it, put ALL your archers in a row at one set of ladders, climb half way up with shield out to protect from occasional sneaky archers. Let your archers take out the enemies at the top of the ladder till they sound a retreat, get onto wall, run down to gate (its undefended cause you have no ram). Open the gate, let your soldiers in that are waiting just outside.