In the background is a wall
Here is another small enclosure in RI, in an area with many cairns, as well as building foundations and quarries.
Here is one in Sharon, MA, near an old cabin. This one is constructed of stacked cobblestones and decorated with a rusty oil drum.
And here is what these might have been. If this wooden outhouse was removed, the outline of bricks and stones in the foundation would be very similar to those above. The door side of all these enclosures is open.