Yes, that's a fort, 'Devonshire Redoubt' circa 1600's. The enclosed body of water is Castle Harbour and used to be the anchorage for the fleet back in the day. The roots of the trees were breaking them apart.
It is a bit dry at the moment, haven't had a decent rain for a couple weeks, but that island is one of the outer ones and is smack on the south shore with nothing between it and the US in one direction and Europe in the other so it is typical windswept rocky coastline with coastal hillside, typically trees are low stature with prostrate shrubby understory. Would have been small bermuda cedars, palmettos, buttonwood and one or two other small trees, then snowberry, sea-ox-eye, goldenrod, prickly pear (found lots of that!) and a limited range of other stuff. The tall trees there now are all casuarinas, invasive and do not respond to windshear hence the proclivity to blow over in storms and take chunks of cliff with them. I was kicking slabs of loose rock into the water all the time!
I treated EVERY stump with Garlon herbicide to prevent regrowth, but there is so much duff and seed still on the cliffs that there will inevitably be some seedlings coming on in a few years, I'm starting to think about how we can treat them with less effort, not let them grow so big will be one! Spot application of herbicide pehaps, when the seedlings are still very small, or just pull the little buggers out, if you get 'em early they do come out.
They are also going to remove all the mature casuarinas from the top of the island to remove the seed source, that will take another summer at least. The boat landing was the volunteer group from the local power company, they were there to drag brush to the chipper for the other crew that was cutting up top and did a great job too.
Thanks for having a look, thanks to Gary for sending the saw, it was a fun summer job, hard but fun!