Jungle Rock Island was most likely a military establishment for the Japanese Imperial Army at some point during World War 2 (1939-1945), and these easter eggs helps to further reinforce these findings. Evidence #1: Pillbox Bunker Approximately 2 are found within the map. Both of which are perfectly circular in shape. Even the map editor refers to these as "japanese bunker." The fact that only 2 are found on the map indicate that the others may have been reclaimed by nature, or worse, demolished entirely. What are pillbox bunkers? Pillbox bunkers, known as pillboxes, are concrete dug-in guard posts, normally equipped with loopholes through which to fire weapons from. They are in effect a trench firing step hardened to protect against small-arms fire and grenades and raised to improve the field of fire (fof). Evidence #2: Flak Tower Remnants (Truck included to provide scale). Flak towers are eight complexes of large, above-ground, anti-aircraft gun blockhouse towers. Flak towers had 20mm 2cm FlaK 30 anti-aircraft weapons and 128mm guns to defend against RAF and Allied aircraft. These towers were used by the Luftwaffe to defend against Allied air raids during WWII. They also served as air raid shelters for tens of thousands of local civilians. The fact that only this much of the concrete tower remains indicate that it was most likely a Flaktrum VII (see below for reference) tower that has unfortunately fell into disrepair or entirely destroyed during a possible heavy bomb attack or any weapons attack capable of causing severe damage to concrete structures. A Flaktrum VII style tower. Hope these help you know more about JRI's activity and history.
I think there is more to suggest that structure is an old fortress than a flak tower. For one, it is brick not poured concrete, it seems to be situated right on and built into a rock outcropping, if it were a freestanding Flak tower, it would take millions of years for that kind of rock structure to build up. We all know how that is unrealistic. Flak towers do not really have merlons, but that structure does. If you look, at the way it's defenses are situated, it is pointing it's side of best fortification at the sea, so it must be a fortress against sea invasion. It has good elevation and it is on a small peninsula, ideal location for a fort to watch the sea for enemy ships trying to enter the bay.
I think your are right. It seems more like an old colonial (Spanish, English, Dutch, Portuguese...) fortification, reused by the Japanese. This one is in the Philippines: https://en.wikipedia.org/wiki/Taytay,_Palawan#/media/File:Fort_Sta._Isabel_in_Taytay.JPG
