This is a 15-panel panorama of the Japanese Bunker one has to climb over to get to and from the wave-watching areas. The holes along the top of the far wall were obviously for reinforcing beams to hold the roof up. While you can't see it in this photo, the roof is made up of probably 2 or 3 feet thick natural rock / concrete with now-rusted metal reinforcing rod (I suspect that the beams extending from these holes were probably wood). It seems quite stable and firm, even without the reinforcing beams. It does give me a bit of pause, however, whenever I am walking by or over it. There are some distortions in this panorama, but keep in mind that this probably takes in about 270-degrees or so. The rock on your left is actually closer to the wall on your right than is the wall with the window.
For more photos around the bunker, see the following:
From JUNE 2, 2012: SUNRISE WAVES EAST OF COW TOWN BUNKER