Mystery fort and mini tunnel [????- ]
The marker is for the fort/pillbox, whereas the mini tunnel is just below it.
It's composed of a mini tunnel, a part of wall which we often seen at pillboxes, then comes two thick walls that look like it's been built above a boulder at one end, and some bricks on the other end.
Not sure to whom do these ruins belong to, although the mini tunnel looks a bit like the one at PB411.
There are some indents on one of the thick walls, with a wide patterned band along the top of the two walls, I haven't seen this type of design before.