A courtyard does not fire heavy arrow fire on the enemy on it's own. Walls do.

I keep the walls defended because I like having my towers get off free kills. Plus one time the enemy assaulted my stone gates with a battering ram. The battering ram never got to the gates before being destroyed by the gatehouse towers. When assaulting a wall using ladders and siege towers, it's usually 1-5 attackers at a time versus the entire defending unit. All these factors, plus the ability to defend a small hole left by a sapping point using a unit of phalanx makes me always defend the walls to the death easily. Hardly anything gets through. If they do, well, I always do enjoy city fighting once in a while.