North Base Tower, Ballymulholland, Magilligan, Limavady, Co Londonderry

The North Base Tower at Ballymulholland is a circular tower constructed between 1820 and 1839 as part of the first Ordnance Survey of Ireland, under the direction of Lieutenant Colby. It played a vital role in establishing a triangulation network alongside similar towers referenced as HB02/09/008 and HB02/10/012. The tower comprises a circular basalt wall approximately 6.0 metres in diameter, standing to an average height of 900mm. It is finished with a sandstone coping and surmounted by a metre-high, white-painted cast iron railing. The structure is of industrial archaeological interest and holds authorship, regional, and international significance. It is currently owned by the local government and retains its original function as a tower.