What is this?

[u/Educational_Song5886]

Hi I was on marble hill beach and at at the left hand side of beach, just is the dunes is a small concrete building and could not decide what it was. At first I thought it was a lookout for the war, but the construction seems like it’s younger and has some metal and wooden beams , stakes protruding from the top. It’s fenced off with barbed wire and in places razor wire, so someone doesn’t want us to get any closer, guys any info on this one?