bcd - watchtowers

The WW1 [right] and WW2 [left] observation posts. These would watch for approaching ships, using rangefinders to plot their co-ordinates, which would be passed to the guns / searchlights in case defensive action was needed.

The hexagonal WW1 building is topped by an armoured steel turret housing a Barr and Stroud Horizontal range-finder, which was camouflaged by a false roof.
Interestingly, the WW2 post was originally equipped with a depression rangefinder, but the angles involved were found to be too small for adequate accuracy, so it was replaced with an up-to-date model of the Barr and Stroud equipment, installed on the roof.