Alderstead Fort

The London Defence Positions were a late 19th century scheme of earthwork fortifications in the south-east of England, designed to protect London from foreign invasion landing on the south coast. The positions were a carefully surveyed contingency plan for a line of entrenchments, which could be quickly excavated in a time of emergency. The line to be followed by these entrenchments was supported by thirteen permanent small polygonal forts or redoubts called London Mobilisation Centres, which were equipped with all the stores and ammunition that would be needed by the troops tasked with digging and manning the positions.

Source: Wikipedia