The London Mobilisation Centres were built along a 70-mile (113-kilometre) stretch of the North Downs from Guildford to the Darenth valley and across the River Thames in Essex. Thirteen sites were chosen, at Pewley Hill, Henley Grove, Denbies, Box Hill, Betchworth, Reigate, East Merstham, Fosterdown, Woldingham, Betsom's Hill, Halstead, Farningham and (to the north east of London) North Weald. The design of each site varied, but they were never very elaborate, just a magazine and storehouses for the mobilisation of troops, with limited defences. The intention was that the centres would, in addition to holding ammunition and other supplies, act as strong points in an almost continuous line of field fortifications. The trench lines joining the Defence Positions could be rapidly excavated on the outbreak of war. Related stores were set up at Tilbury, Warley and possibly Caterham. They were quickly viewed as obsolete, and all were sold off in 1907, with the exception of Fort Halstead, now used by DSTL.