Castlerigg sits on an ancient track from Borrowdale, a source of stone for axes. The ring has a flattened arc at the north-east. It also has an entrance defined by two two tall stones flanking the gap at the exact north. Around the circumference are traces of a bank.

Thirty-eight stones are placed in an slightly oval shape of 30m (100ft) in diameter; a further 10 small stones are arranged as a rectangular enclosure on the south-east side of the ring: this is a feature unique to Castlerigg, nothing similar being present in other stone circles. The largest stone of the circle, not far from the enclosure, is 2.5m (8ft 3in) high and it weighs about 16 tons: most of the others, much smaller, are 1 to 1.5m (3-5ft) high. At the north of the ring is an entrance marked by two slightly bigger stones, and about 90m (295ft) to the south-west, by a stile at the edge of the field, is a single outlying stone, 0.9m (3ft) high.
 

There are many theories about Castlerigg's function. In Professor Alexander Thom's opinion, the circle was an astronomical observatory, while Professor Aubrey Burl wrote that one of Castlerigg's many functions may have been to act as an emporium connected with the Neolithic stone axe industry in the Langdales. The close mountainous source of the tuff used for such tools and the stone axes found at the site support this theory. Probably, Castlerigg had a variety of functions: easily approached from all directions, it was probably used for trading, religious ceremonies, and tribal gathering. The rectangular enclosure was excavated in 1882, and only charcoal was found. No other excavation has taken place, either within the enclosure or outside.

The site was first brought to public notice in 1725 by the antiquarian William Stukeley, who wrote that the circle was very entire, consisting of 50 stones, some very large. But in 1849, in his Guide to The Lakes, Jonathan Otley reported the present total of stones. Castlerigg, known to local people as Druid's Circle, is also called Keswick Carles, apparently because of an old legend telling that the stones are petrified men, but in fact for a misunderstanding of William Stukeley's word Carles for Castle.