The main compound and tool store of East Merstham London mobilisation centre, known as Alderstead Fort, situated on a chalk ridge of the Surrey Downs. The main compound forms the south western part of the monument and houses a three-roomed magazine fronted by five casemates. It survives as a roughly north west-south east aligned, north east facing, flat-roofed structure constructed mainly of reinforced concrete and brick, with a large, crescent-shaped earthen blast-bank partly covering and flanking the magazine to the rear. Beyond the bank is a ditch up to 10 metres wide. The building retains many original fittings, including iron-and-wood door and window shileds and magazine vents. The associated red-brick tool store lies around 75 metres to the north east and is a north east-south west aligned, single storey, rectangular building measuring 14 metres by 10 metres, with white washed sandstone dressings under a pitched, blue slate roof. The south eastern face has full-length, central double doors flanked by tall windows with iron grilles. The original caretakers lodge, situated around 40 metres to the north east, is in use as a dwelling and is therefore not included in the scheduling. Scheduled.