A structure built into the wall

On my topo map of Westford there is a hill marker "Decator" which turns out to be the "Cider Mill" conservation land. I explored it a bit and found nothing significant. But there was a wall running across it, north to south, with a curious structure built into it on the northern side of the hill. Here is another view:
It is like a kid's fort but it seems too well made.