The monument at Threecastles Demesne near the River Nore is the kind of site that only gives itself away from the air, where the geometry of its construction shows up as a cropmark, a faint discolouration in growing grain caused by the differential moisture retention of buried earthworks beneath the soil.

The monument was first identified from aerial photographs taken in July 1971 and again in July 1973, and what those images revealed was surprisingly elaborate. The structure is a multivallate enclosure, meaning it has multiple concentric rings of ditches and banks rather than a single boundary, a design associated with high-status prehistoric funerary or ceremonial use. At its centre is a roughly circular area approximately 22 metres across, possibly the remnant of a burial mound, surrounded by a berm, a flat ledge of ground between earthworks, then a wide fosse, or ditch, then another berm, another fosse, and finally an outer bank and ditch. The whole complex spans around 100 metres in diameter. When the site was visited in 1987, the surviving remains were extremely low, the central platform rising only 0.4 metres above the surrounding ground and the outer bank barely 0.25 metres high. A field boundary cutting across the south-western quadrant had already truncated part of the outermost ditch, erasing what had once been a complete circuit of enclosure. The site occupies a position with clear views along the Nore valley, which suggests the location was chosen deliberately, the land rising northward to a ridge only 370 metres or so from the river.