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 the National Archives at Kew are two rare photographs documenting the navvies, or unskilled labourers who worked by hand to construct the impressive earthwork defences that remain today.
While the navvies were unskilled, but as this was a military construction project it is likely that there was some Royal Engineer supervision. If you look to the rear left of the image you can see two men wearing what appear to be a Royal Engineers belts. See the image below for a comparison.