I'm having trouble understanding gatehouses. In Zhengzhou (the fourth mission of the Shang campaign), I made the same decision as in the walkthrough (to put the farms to the southwest of the city and to open a gate in that wall for delivery), but had trouble with gatehouses. Nothing seemed to work. I had nothing in the 3x5 area between the walls, but it was still a red footprint. I had a road down the middle, but no luck. I had walls all the way across, no luck. I had walls except at the road, no luck. I had roads up to the edge of the 3x5 area, no luck. Tried a one space gap in the road, no luck. Even tried deleting one of the existing gatehouses (left a five space wide gap in the wall with a road through), but couldn't replace it. Tried all this again towards the end of the mission (after palace and AC was built, for example), but still couldn't get any of the above to work. I realize I'm probably missing something stupid; anyone want to tell me what?