On this subject, let's do a mechanical ruling/clarification which might well become important in the very near future.
Fortification Combat
Fortifications (keeps, castles and to a lesser extent city walls) all provide bonuses to defenders that hold them, provided they are on the defensive. A standard motte and bailey fort (like Windheath's starting forts) grants a +50% defensive strength bonus to defenders. City walls grant +25% defensive strength bonus to defenders. Higher level forts grant an additional +50% strength bonus per level.
Cavalry lose their plains bonus when attacking units in fortifications unless the fortification has been breached by siege equipment. They still retain their tactical and general strength bonus, but horses aren't great for attacking walls.
Bridges
Bridges cannot be breached, but they can be destroyed. Bridges are special in that they force a maximum of three melee units per side to engage because of the width of the bridge. Ranged units (archers) may continue to take part in the fight as normal on either side, but generals must choose which three units (or less) of their army to stick on the front line and fight for the bridge.
Collapsing the bridge (via siege engine damage) will destroy all units fighting in melee on the bridge.
Bridges can be burned by armies that are present at the bridge, provided they are unassailed by other armies and provided there is not a hostile fort next to the bridge (e.g. in the case of Westwatch). In such a situation, fort defenders sally forth and kill the engineers until the fort is taken or otherwise neutralised. The best way to secure a bridge is to build a fort next to it.
Garrisons
Cities and forts (but not bridges) have an immovable garrison of 1 standard regiment, no matter what other forces occupy them. This regiment is replenished after a fort is taken by force and is always there to provide a certain degree of resistance even if the rest of the army is off fighting elsewhere.