In two battles, I've had the attacker (Spanish in one case, Prussian in the other) send troops into the fort buildings to melee with the defenders while other units ran for the flagpole. In the Spanish case, two enemy units went in to melee with my grenadiers in the small armoury, only to be both broken because of exhaustion and casualties.

I've played around with the cheval-de-frise:
- on the 'side' of the front corners of the fort. It doesn't do much to infantry but if a side/rear gate opens and the enemy cavalry makes a run to get in, they'll take the shortest path and end up as horse-kebabs...
- across the inside of a couple of the entrances (especially the rear gate as the AI likes to run around the entire fort first...) so if I let them take the gate, they'll open it and try to surge their cavalry into the fort with the same result.

Cheers,
Dreadnought2