They built a house under the map. Here how its done. 1. build a door near the cave wall and open the door, it will push you through the wall. 2. Then you can walk there a bit, and build a foundation on the outside, which is under the map. You can build a whole base under the map, cupboard , whatever. 3. those base are almost unraidable, except from other people who are also under the map, probably using another cave as a point of entry.
I was playing under the map as i was trying to destroy a wall around radtown. I learned that the wallers also had a base under the map and they raided me under the map, and placed cupboards around their radtown walls, under the map.
I wish this would be adressed because its ridiculous. the whole gameplay is ruined on our servers because most of japanese player (i live in asia) build inside rocks / under the map.
Edit, this still works as of latest updated i tested it. the “tweaked buildings to prevent building inside rocks” only prevented foundation to be placed inside the rocks, but anything else hasnt changed, and under the map also hasnt change it can still be built.