This brick-built building overlooks the beach and measures 7.5m by 4m. It consists of two compartments, the northern open at the rear, the southern an enclosed room with a single entrance door. The roof is constructed of reinforced concrete. Both bays of the brick-built building contains observation slits facing out over the sea. The slits are defined by re-enforced concrete mouldings. Both slits have been bricked up. The building has a concrete plinth adjoining on its northern side whose surface contains post holes for a structure now removed. Fifteen metres to the east is an associated BCF hut (see NPRN 506504).

Event and Historical Information:
A minute in the wartime papers referring to the merits of maintaining the military research site at Walton-on-Naze, Essex, includes a reference to the desired building plan for observation posts at the Experimental Establishment at Ynyslas. 'The actual floor area does not appear to be important? length of the observation slit should be sufficient for about 20 spectators. This will probably mean a total length of about 20ft allowing for double banking. Building to be constructed of splinter proof construction. OP should be situated about 30 yards to the rear and 230 yards to the side of the projector, lighting and heating should be provided with a separate power to operate camera. Observation slit should be so arranged that the round may be followed when fired at max QE, which may be as high as 70 degrees. Fort Halstead, 8 February 1945.'

Sources include:
Dyfed Archaeological Trust HER, PRN 31465
T/407/OP facilities at Ynyslas - scheme 2 with ref to minute 25 January 1945, The National Archives, Kew

Maritime Officer, RCAHMW, December 2009.