(some post above have already mentioned most of this)
The best way ive found is to have a unit break down the gate then you can either march a phalanx into the gate or have 2 or 3 units rush in (ive found that calvalry or units with very high dmg that dont rely on each other like gladiators are better than phlanx units at rushing a gate).
Once you have the gate make a 'box' where you have 2-3 units facing down both streets with a little space in-between so they cant get flanked, and dont rely on the fact you can have your phalanx turn around to face oncoming units instead of making a box cause if the enemy has both ends of the street occupied your screwed.
Then like hussar said slowly march through the streets and let the enemy kill themself on your wall of spears, dont make the mistake of making a 'wall' at one end of the streets and marking your flanks off as safe and forgetting about em, move your forward-facing line up a little, then your backwards-facing line up behind them like leap-frog. your shouldnt have any problem.
Unless the defenders has mass calvalry rushing your 'wall' together, if so I suggest making 2 rows instead of one, mass calvalry will take huge casualties rushing a wall but they can often break it with bodies.
It takes twenty years or more of peace to make a man; it takes only twenty seconds of war to destroy him-Baudouin I
War is Hell - William T. Sherman
To call war the soil of courage and virtue is like calling debauchery the soil of love - German Proverb
All warfare is based on deception - Sun Tzu[This message has been edited by Praetor Zankaru (edited 04-26-2009 @ 10:25 PM).]