Possibly a 75mm AA gun position. If so, the indentations in the walls are for ready ammunition and most likely built by the Japanese soon after they captured the island. I have also seen this here on the west coast for the 90mm AA guns that were in service in the early 1950's, but slightly different in configuration.
The 75mm was manually trained and not effective at higher altitudes
Good to see somebody is photograhing these.
As an amature historian, I got curious and did a look up of some WW2 photoes of this island. These for concrete bases supported a large and possibly multi tierd observation tower that faced north. Given the history of this area and the battles fought there, it is my guess that a number of wrecks of various sizes and types lay deep in the waters off these islands.
Nike Site SF59L missile prep building
SF 89 Tracker Site
WW2 155mm GPF Mount #1
AC&W Operations Building