The Old Grammar School, on Castle Street in Castletown overlooking the harbour, is described by Manx National Heritage as the oldest roofed building on the Isle of Man, dating to around the 1200s. It was originally built as St Mary's Chapel, serving the medieval community that grew up around Castle Rushen, before Bishop Thomas Wilson converted it into a grammar school around 1710. For nearly two centuries it educated boys, predominantly from wealthy families, and its former pupils went on to become Members of the House of Keys, judges, and lawyers. The school closed in the 1930s, and in 1950 the building was saved from demolition when the surrounding medieval streets of old Castletown were cleared. It is now open to the public as a small museum, sometimes visited as part of Castletown Heritage Tours.