A Fortress is a large wood or stone structure usually with cannons and housing. They replace walls in the earlir games and expand the system of Midieval Total War. Historically, the were build around towns or in key areas to halt enemy forces. It is important to remember that anyone can occupy your fort, you, or the enemy.
Defending from a fort[]
This is in fact very easy, but if the enemy has artillery then you will have to think fast and be aggressive. You should have infantry stationed on the walls and at the gate, this way, any enemies will be under fire before even getting to the walls. You should have Cavalry outside to lure your opponent into cannon range or to chase retreating units. As mentioned, you must be aggressive if the enemy has artillery, Cavalry is well suited for this task. Should a enemy breach or destroy the walls, you have two options: defend, or counter attack. Defending strategies are garrisoning infantry into the internal buildings. For the latter option, bayonet charges and Cavalry charges can scatter a enemy fairly easily. These general rules can ensure victory for most defending battles.
Attacking a fort[]
Attacking is a lot more stressful then defending, as it can easily lead to defeat or severe casualties if your not careful. It is good to have plenty of Infantry and a unit or two of Cavalry. One thing to always have is artillery, the more the better. You should set up you artillery and destory the walls of the fort. If your lucky, the enemy will then charge at you, all while your artillery weakens them. If they don't move, attempt to lure them out. If by some odd circumstances you can't get your hand on any artillery, you'll have to try a different approach. You must get Infantry to the walls, they can then scale it, fight the enemy on top, and fire upon the enemy inside the fort. Though this stratagy can lead to extreme losses.
Another Strategy, although rather time-consuming, is to flank the enemy. Send several strong infantry regiments to a less defended wall, and scale it. then once you have captured the gatehouse, open the gates and rush your forces in. this can be used on any number of walls, and can lead to quick and decisive victory.