Part of the Elan Aqueduct (nprn 95469), itself part of the Elan Valley Water Scheme (nprn 96459), Knighton Tunnel is about 3.9km long and carries the aqueduct under the uplands between Monaughty and Knighton. A pair of observation towers (nprns 403806 (East) and 412105 (West)), presumably used during the construction of the tunnel, stand on Black Hill, 332m apart and on the alignment of the tunnel. Each tower is constructed in stone rubble, about 4.5-5m high and about 1.5m square in plan; the upper section is smaller in plan and has on top a fixed iron frame for mounting a surveying instrument. The towers are erroneously annotated on the modern large-scale Ordnance Survey mapping as Air Shafts.
B.A.Malaws, RCAHMW, 16 November 2010.