A report made following a 1933 visit to Castle Howe (contained in the National Monument Record) indicates that part of Castle Howe may be prehistoric:
The lower chamber appears to be prehistoric, its walls survive to a height of 5ft. 3 ins; the stairs may be original...”
A similar report following a visit to the site in 1964 makes no attempt to date the site. There is no certainty as to the age of this monument, but the NMR implies that part of it may date back to the Iron Age.