The view from inside that Battery Commander's Station! That pintle in the center would have had a big pair of binoculars affixed to it, if it were presently 1901, which I assure you that it is not. This looks a whole lot like a similar Battery Commander's Station at Fort Mott in New Jersey in which I ate lunch whilst visiting Fort Delaware and environs in 2012 (see below).

...except New Jersey obviously has the money to keep up its antiquated military structures, whereas Massachusetts does not.