Whitley Castle is situated about 1,000 feet (300 metres) above sea level in the Pennine hills on the border of Cumbria and Northumberland. It lies to the west of the modern A689 road beside the Pennine Way long-distance footpath. It is one of the most isolated Roman sites in Britain, which may help to explain both why it remains largely unexcavated (in 2012), and why so much of it has survived. The site is a lozenge-shaped spur of high ground on Castle Nook hill farm under permanent pasture. The remains lie under the grass, and are most clearly seen in aerial photographs. The Roman fort itself covers about 4 acres; outside it is a system of concentric defensive ditches.