When you select the range of squares to build a farm field, it doesn’t invalidate the entire field just because you have a building in the range. It marks those squares red and just continues to build around them. The bug, as I see it, is that when you try to move a building onto a farm field, it doesn’t offer to reclaim the land that the building needs from the farm field. Moving a guard tower in the middle of the field should be no different than erecting a scarecrow in the middle of a corn field. We don’t have tractors to drive around. All the tilling is done by hand, so it should be no problem to till around the legs of a tower. Just the same, deciding to move an entire building onto the field should at the very least be allowed with a warning. Otherwise, it just seems inconsistent that you are allowed to build a field around a building.
By the way, I know we don’t have corn or scarecrows, yet.