No-build zone from hillside comes adjacent to the control point trucks eastern boundary and continues uphill to the extent of the control point grid. Result is a 3 block gap in defendable perimeter dangerously close to the control point truck.
My initial thought was the slope, but another significantly steeper face of the same hill is able to accept constructions (which I find funny, hence that inclusion in the picture). Affected area is of mild slope and appears to be free of obstacles.
Xbox one, solo.