Prior Castells Tower was originally a four storey stone tower house, founded in 1500 by Thomas Castell, Prior of Durham. In 1255 to 1536 the Monastery of Durham, established the House of Farne, a small Benedictine monastic cell on the island. Originally encased by a stone precinct wall, the entrance was into the irregular north court but now you go into the east court, between the chapels of St Cuthbert's and St Mary's. In the centre court is an hospitium and the tower which appears to incorporate earlier work and its low round headed doorway leads into a tunnel vaulted basement. Garrisoned as a coastal fortification from 1559 to 1637, from 1673 King Charles II, granted a license to erect a beacon and the tower became a lighthouse. In the mid 19th century, Archdeacon Thorp inserted Gothic windows and redesigned the interior to make the tower, three storeys.