In its original form - as the chapel for the growing community of Castletown - the building was rectangular and probably measured about 12 by 7m. The latest interpretation of the much altered building suggests that the arcade forming the south wall is the earliest surviving element, and is probably of later 14th century date.


Masonry scars suggest that both gables are later rebuilds onto the south arcade, slighlty outside the line of their predecessors, and thus adding to the length of the building by 1 to 1.5m. The gables are now pieced by large rectangular openings which have substantially removed traces of their predecessors.


It follows that the north wall is also later. The present doorway lies in the north wall and appears to be original. Two window openings, one each to the east and west of the doorway, are now blocked, and a chimney flue has been inserted just to the east of the door.


As early as 1584 the building was functioning as 'the Scholehouse', for which these alterations, and the addition of the annexe to the south, were undertaken. It finally went out of use as the grammar school in 1930, and is now a museum.