The site of an early medieval keeill or chapel, which would have been in use between circa AD500 and AD1000. This is almost certainly the site of St Mary's Chapel, Castletown, in the parish of Malew - the early-medieval predecessor of the building later known as the Old Grammar School, Castletown (grid reference SC265674). Manx National Heritage's iMuseum catalogue lists three linked records at this Castletown site: an Early Medieval "St Mary's Chapel, Castletown", a Medieval-period "St Mary's Chapel, Castletown, Malew", and the 17th-century "Old Grammar School, Castletown, Malew". A.M. Cubbon's 1971 study 'The Mediaeval Chapel of St Mary's, Castletown, Later the Old Castletown Grammar School' records the chapel as founded in 1250, and J.R. Bruce's 1968 Manx Archaeological Survey speculates that the building's arcade arches may be reworked stonework from Rushen Abbey. No excavation or finds data specific to the early-medieval keeill phase itself was located - only the later medieval/1250 chapel's architectural history.