Interesting, it looks like a horseshoe sheep fold with a straight stonewall "bar" across the open front, leaving only 2 small entrances. Then surrounding it is a larger rectangular enclosed space, looks like stone walls on four sides. However, it's on the highest point around, on rock, an unlikely position for sheep herding/use.
It seems quite dark in the centre of the shoe. Might it have been a old form of lime kiln to burn limestone to produce quicklime?
There are a lot of round holes in the area nearby, presumably either sinkholes or man-made excavations. That shape protects the fire/temperature from the wind, and the two entrances enable rudimentary fuelling and air control to a fire. You'd want it high up due to the fumes. This link seems to (sort of) support my cock-eyed idea.
There are some sketches of similar shapes both Roman, and later farming agricultural clamps (to produce fertiliser). Similar to charcoal and brick clamps but possibly not with a closed in roof, all pre-industrial.
So your mission, should you choose to take it...
looking for any signs of burning, random chunks of burnt or unburnt limestone, or a lot of stone around it to build/strip away kiln walls. ?
Later more industrial lime kilns: Lime kilns at Herbert's Quarry or the Black Mountain Quarries in the western Brecon Beacons Bannau Brycheiniog
https://heneb.org.uk/archive/dyfed/calch/index.html