Located outside the wire on Crow Drive, building N17 was constructed as an optical test range. Most likely originally constructed entirely from wood, the later edition of a brick front facade and roller shutter door may have come from a change of use. By 1975 the building was under the control of the admin department
Constructed as part of the London Defence Scheme, the fort at Knockholt (also known as the Polhill Fort and latterly Fort Halstead) was part of an extensive earthwork defensive system constructed on the North Downs. There were a total of thirteen mobilisation centres (also known as redoubts or forts) constructed in the late 19th Century.
In 1989 a report was published aimed at “drawing together at the current time, information regarding the physical plan, form and history of the ancient monument known as Fort Halstead.” These rare images detail some of the historical features still remaining in the old mobilisation centre almost 100 years after its construction. At the time,
“In this building a group of scientists & engineers led by the then Dr WG PENNEY worked on United Kingdom atomic warheads during the period 1946 to 1952. This plaque was unveiled on 6 APRIL 2982 by LORD PENNEY OM. KBE. MA. PhD. D Sc. FRS.”