Whitley Castle (Epiacum) is a large, unusually shaped Roman fort (Latin: castra) north-west of the town of Alston, Cumbria, England. The castrum, which was first built by the Roman Army early in the 2nd century AD, was partly demolished and rebuilt around 200 AD. It appears to have been sited to protect lead mining in the area as well as to support the border defences of Hadrian's Wall.

Unlike most Roman forts that have a "playing-card shape" (rectangular with rounded corners), Whitley Castle is lozenge-shaped to fit the site. Numerous banks and ditches ring the stone ramparts, making it among the most complex defensive earthworks of any fort known in the Roman Empire.

The site was surveyed by the geologist Thomas Sopwith in the 19th century and the historian R. G. Collingwood in the 20th century. In 2012, a geophysics survey was conducted by a team from Durham University but it has not been fully excavated. Among finds at the fort are altars with inscriptions to Hercules by Legio VI Victrix (normally stationed at Eboracum [York]) and to Apollo by the 2nd Cohort of Nervians, the garrison of auxiliaries. Other finds include a midden containing shoes; coins, fragments of Samian pottery, beads, nails, and a bronze handle shaped like a dolphin.

Although its English name is Whitley Castle, the Romans knew the fort as Epiacum. It is named in Ptolemy's Geography as the first town in the lands of Brigantes tribe in northeastern England. Its Latin name probably comes from a local British name *Epiakon "the property, or estate of Epios", Latinised as Epius. Epios may have been a local leader or chieftain of the Brigantes tribe.

Whitley Castle is about 1,000 ft (300 m) above sea level in the Pennine hills on the southern edge of Northumberland near its border with Cumbria. It lies to the west of the modern A689 road beside the Pennine Way long-distance footpath. During the Roman era, Epiacum was situated about 15 miles (24 km) south of Hadrian's Wall and 20 miles (32 km) north of the main road which ran between Luguvalium (Carlisle) in the northwest and Eboracum (York) in the southeast.

Whitley Castle is one of the most isolated Roman sites in Britain, which may help to explain both why it remains largely unexcavated as of 2018 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 for sheep. The fort remains lie under the grass, and are most clearly seen in aerial photographs. The Roman fort itself covers about 4 acres (1.6 ha); outside it is a system of concentric defensive ditches.

The fort may have been sited to exert control over the area near Alston and its lead mines, as well as to provide support for Hadrian's Wall.

Epiacum was built early in the 2nd century AD. It was at least partly demolished and rebuilt around AD200; the destruction coincides with an uprising of the northern tribes in 196. The fort was modified or wholly rebuilt about the year 300. It appears to have been preceded by an Iron Age fort, followed by a Roman camp before the permanent fort was constructed.