Britain's Last Line of Defence: Pillboxes, Stop Lines and the Sea Lion Coast in the Defence of Britain Database

In 1940 Britain built thousands of pillboxes, anti-tank obstacles and inland stop lines against an expected German invasion. The Defence of Britain Project, a volunteer survey run by the Council for British Archaeology from 1995 to 2002, recorded what was left. I use its open database, 14,690 located site records, to ask where the defences were, how much was recorded as standing, and whether they were concentrated where the German plan, Operation Sea Lion, intended to land. Pillboxes make up 55% of records. Of sites with a known condition, 71% were standing: 81% of pillboxes but 12% of beach and landing obstacles, mines and flame traps. 90 named stop lines hold 37% of sites, and their sites survived better than others (81% against 65%). Within 10 km of the four landing beaches of the plan of 30 August 1940, there are 5.6 recorded sites per km of coast, 2.3 times the density along the rest of the south and east coast; the result holds at 5 and 20 km. The excess is driven by the beach nearest France, and the densest recorded stretch of coast is Teesside, which points to uneven recording. Only 13% of recorded pillboxes have a pillbox mapped on OpenStreetMap within 50 m today. I add airfields, radar stations and naval dockyards from Wikidata and Wikipedia to place the defences in a wider wartime landscape. The database is a map of what volunteers recorded, not of everything that was built, and I treat it as such. Interactive version: https://tristan-hotham.github.io/stop-lines/. Data and code: https://github.com/Tristan-Hotham/stop-lines. Blog post: https://blog.tristanhotham.com/2026/10/28/stop-lines/.