Defending forts/cities

[Kekvit Irae]

I find that defending towns is easy if you garrison each building with one line infantry or melee infantry, and keep one infantry or cavalry unit nearby to fire on or attack whoever tries to storm the house.

Same thing for siege defense. Though, at the start, line up your infantry along the walls at first to get a few free kills, and then retreat them down to the garrisons when the enemy starts scaling. When the enemy takes the walls, the gates will open for the rest of their units (this is a good thing). Have your cavalry standing by to charge into any enemy that comes through. The enemy will most likely be at least Tired by the time they march through the gates because most of the time they will be running. This will give your cavalry the advantage.

EDIT: Also, having cannons with canister shot will greatly help in the defense of a fort. Aim that sucker at the gate and watch the enemy fall in droves.