Part of the Elan Aqueduct (nprn 95469), itself part of the Elan Valley Water Scheme (nprn 96459), Dolau Tunnel is about 6.8km long and carries the aqueduct under the uplands between the villages of Dolau and Bleddfa. A pair of observation towers (nprns 413072 (west) and 413074 (east)), presumably used during the construction of the tunnel, stand on the route. The east tower stands in a forestry plantation and consists of a substantial round masonry tower, two storeys high; at the top is an iron frame for mounting a surveying instrument, accessed by an internal iron ladder.
See for comparison the Observation Towers (nprns 403806 and 412105) on Knighton Tunnel.
B.A.Malaws, RCAHMW, 17 February 2011.