Visible crew/equipment: On Skull Island, when Carl says, "No one has lived here for hundreds of years," the distinctive reflection of the reflector screen is visible in the lens of Preston's glasses, as he looks down; there are other moments in the film where this happens with characters' eyeglasses.
Visible crew/equipment: As one of the Venture crew members picks up a chloroform bottle from a case, you can see a bright studio spotlight reflected in the bottle.
Chosen answer: The original creator of King Kong, Merin C. Cooper, wrote a novel adaption of the movie in which it was explained that the gates were built by a earlier culture of islanders that were friends with the "Kong" race. The "Kongs" helped the original islanders to build their village and the wall (thus meaning the gate had to be big enough for the giant gorillas to walk through). By the time of the events of the movie, the original islanders have "died out" and their old village had been taken over by a race of more primitive natives who became enemies with the Kongs, and were trying to use the gates for safety.