Pardon if this has been suggested, but I looked and cannot find it.
I find this discussion on whether or not the thing has been suggested before to be totally unnecessary and completely redundant. I searched. I even read all of the options on the permanent voting software. Nothing. That's why this thread exists. End of discussion please, thank you very much.
On the subject of gates, they could easily be put in as a building (like floor bars or gem windows in that respect) or, perhaps less easily, as a piece of furniture (like doors).
Most importantly, Wagon pathing would need to ignore them for this to work. Regardless of whether or not they are closed. If a wagon cannot CURRENTLY go through a gate, they will patiently wait outside until the gate opens. Or, optionally, wagons and friendlies can ALWAYS go through a gate, and invaders must break it down in order to pass.
Each gate might serve as a station for two Fortress Guards. That's enough to help out with any altercations at the gate without being a military force. To prevent abuse of this, a gate must be stationed adjacent to a wall.