Scapa Flow
The natural anchorage of Scapa Flow became famous as the home of the British Fleet in two World Wars when servicemen filled the islands.
The islands were ringed with concrete gun batteries such as this one at Hoxa on South Ronaldsay.
More peacefully, a Nissen hut was converted by Italian Prisoners of War to a camp chapel with ingenuous use of found materials and concrete, to form the Italian Chapel.
During the Napoleonic Wars two Martello Towers (of only three in Scotland) were built on Hoy to protect shipping from American privateers. The one at Hackness is in the care of Historic Scotland and can be visited.