Like everyone else, I try to either jam the gate or form the pocket depending on what the attacking army has coming at me.

The only variant I do to this is:

If the enemy has siege artillery

and I am defending a settlement which has a gated inner keep type of area.

then I will pull all troops back to that gated area.

It has been my experience that when the AI brings siege artillery, the will use it whether they need to or not.

The first time I experienced this I had decided to fall back at the start simply because my forces were terribly outnumbered, lacking sufficent force to effectively man the gate and the walls both. Especially against ladders and breeches. I wanted to have more control of the engagement so I immediately ran all my forces back to the gated inner keep area.

I noticed while doing this that the AI sat out there happily chucking big rocks at my walls and making holes, even though I had, to all intents, abandoned the gates. All they needed to do was put up a ladder, take the gates and let everyone in.

Instead they sat out there for quite some time and made no fewer than six holes in my walls.

/shrug.

At the inner keep, I executed the standard - clog the gate with spears, attack with swords and cav, harrass with missiles etc.

Works like a charm! (ymmv)

You don't get the opportunity to hit them from the flanks so much this way but, it's a fantastic choke point. Plus, it's a lot closer to the city square so, if things go badly, routers don't have so far to run ;).