Fort Platte Saline
Guernsey, Guernsey
Apart from a Roman Fort, there were very few fortifications in Alderney until the mid 19th century. These forts were then modified and updated in the mid 20th Century by Germany during the occupation period. Alderney at 8 km2 is now one of the most fortified places in the world.
Fort data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).