I must be doing something very right or very wrong. In a couple years of playing this game, I've only had a castle assaulted a handfull of times. Those assaults always occurred on border provinces where the garrison was pitiful (less than 100 men). I can't imagine a situation where I would be defending a fortress, so I've never messed with the defenses. Like Caravel, I do build ring walls, etc. if I think a castle may be on a border with a small garrison for quite awhile. This is purely to help the garrison hold out longer in a seige.

Does anyone know exactly how much longer a garrison can hold out as a result of a wall?