Really depends on numbers attacking versus my own defending.
My usual frontline cities will have two-three spear militia solely for the gates, and two heavy cavalry (lance wielding preferably). The spearmen will attack and hold the gatehouse when the gate is broken. Usually cavalry units will make it past the spearmen in the initial rush, so I use one unit of cavalry to mop them up. The last unit is either to rush from the side if my defense is breaking, or to move out of a side gate to hopefully attack from the rear once they're fully committed. (aiming at either the general, or taking the gatehouse troops from the rear)
Lastly if I can afford it I'll have one heavy swordsmen unit reading to help the spearmen blocking the gate. But honestly I can rarely afford the extra unit. The cavalry already make this expensive enough of a defense, but they've saved so many cities, especially when my gates have been opened by spies.
Personally I can't stand pikes, unless they're fully upgraded. I find they're slaughtered by any modern infantry unit, and while cavalry are a threat, their charges can be stopped in the gatehouse by the spear militia.
Lastly I'll have my general unit close to the gatehouse (but not attacking), simply because its these units that need the morale modifier the most. Those on the walls already have enough bonuses as it is. The troops on the ground benefit the most from his/her presence.
Note (1). The morale banner unit of the Italian states is an excellent gate defender. Park the unit right at the gates, and the actual equipment will block the gate, allowing only small numbers of enemies through. Trebs also work this way, although for some reason, the AI is more likely to destroy the treb if its in the way.
Note (2) Archer spikes. England and few other archer units (Turks I think) have the ability to plant stakes. I'd recommend trying to plant stakes as close to the gatehouse as possible from the wall. As said before your troops on the wall already have the advantage, so the stakes are best used against the units milling around outside the gatehouse trying to push their way in, & reinforcements waiting to the side. They can do some nifty damage.
Note (3) Those Eastern units (I know Egypt & Turks get them, not sure about other eastern factions) with the fire grenades are awesome for gate defense. just put one or two units at the top of the gatehouse (I like having two units, one for either side), and fire away. Awesome unit. Just try to give them an armour upgrade.
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