Almost all of your questions can be answered with a resounding "Depends..." Every city had walls, of some kind or another. They existed to tax and regulate trade as much as for defense.

The garrison would generally be made up of militia, as the citizens had a civil duty to put in a couple days a week on the walls in peacetime, and everyone turned out during war. Mercenaries were used, but only if there was a pressing need. The kind of men who will kill for money will also open a gate for money. If the town was under the protection of a lord or other noble, he might send his own troops to defend the town. The city's ability to handle a siege depended on many factors, such as the amount of food stockpiled in advance, the number of people inside, and the level of sanitation. If plague broke out, or fires torched the granaries, the defenders would be in a lot of trouble.