Earthwork remains of an Iron Age hillfort containing levelled rock-cut hut platforms, according to ADS, but they're hard to spot amongst the heather. It's quite a small fort, sited on top of a rocky outcrop. There's not a lot to see, but it's obvious that the site would be easily defended.
You can see Castlerigg Stone Circle from the top, if you know where to look, and have binoculars.
It was a cold day of sub-zero temperatures, when me and my mate, Pie Eater, visited. The ice, under the light covering of powder snow, made for an interesting ascent. The Forestry Commission have spoilt the area somewhat, but the short detour to the top of nearby Raven Crag will reward the intrepid antiquarian with spectacular views of Thirlmere.