The ancient earthwork and the medieval tower built within it have ended up sharing a fairway, which makes this one of the stranger layered landscapes in the county.

A promontory fort is exactly what it sounds like: a fortified headland, where a ditch and bank cut across the neck of land effectively turn the seaward projection into an island. Here, that ditch runs in a rough arc across the headland, approximately 84 metres in length, and appears in places to have been hacked out of solid rock. Writing in 1989, Helen Roe calculated that the radial distance from the tower at its centre varied between about 60 metres at the southern end and 71 metres to the north, suggesting the ditch was not a simple straight cut but followed the natural contour of the ground. It probably ranged between roughly 3.65 and 4.57 metres wide, and was perhaps 1.8 metres deep, with an earthen bank thrown up on the outer side only. The medieval structure sitting inside this earlier enclosure, Barrow Round Castle, is a tower house whose main doorway faces inland rather than seaward. Roe interpreted this as deliberate: the sea approach across the Creek of Fenit was the castle's natural defence, while the landward side required the ditch. She also noted that tussocky ground near the tower conceals what appears to be the outline of three sides of a rectangle, possibly the foundations of additional defensive works. The castle and its enclosure were later reused as a bawn, an enclosed courtyard typical of defended tower-house complexes.

The golf course landscaping has altered the ground considerably since both the fort and the castle were in use, and some stretches of the ditch were backfilled before surveys were carried out. Aerial photographs from 1949 and 1962 captured the ditch more clearly than is visible today, but traces of it can still be made out roughly 60 metres east of the castle on the present course.