Hillsborough is the site of an Iron Age hill fort atop the cliff on a promontory at approx 115 metres above sea level. The fort takes the classic shape of a promontory reinforced and cut off landwards by a defensive earthworks.[5] These consist of two linear banks and ditches, oriented northwest to southeast, and contouring across the slope. For an Iron Age fort in northern Devon, the length of these banks, at 280 m (900 ft), is large. There is a clearly defined gateway near the eastern end of both banks, which is likely to have been the main entrance to the interior of the fort. Research using earth resistance and
gradiometer surveys has produced good data, but interpretation is difficult because landscaping of the area was performed in Victorian times.[4]
The Ordnance Survey Map of Britain in the Iron Age shows Hillsborough as multivallate and one of the two largest enclosures in Devon along with Cranmore Castle at over 15 acres, since the size of the enclosure today is less than half this; this must be based on more than half of the enclosure being lost to coastal erosion.[6]
