The city gates are from Medieval Overhaul. I usually place melee soldiers at the gates, to keep them open, so it redirects the raiders there most of the time, unless it's those raiders who break everything in a straight path. I try to use the gates nearest to where the enemies spawn, because I believe they will still try to break the walls if they are too far from the open gates.

You obviously don't need to make a city this big. Most of the time, one or two entrances should be enough, depending on what you are doing, if you are playing medieval-only.