Today we walked along the coast from Aberlady to Yellowcraigs, then through the fields up to Dirleton, passing on the way several evidences of WW2 constructions.  These were built as part of the coastal defences created along the Forth to provide protection in case of invasion from the sea because the Forth was very important especially with Edinburgh nearby and the Naval Dockyard and it was the gateway to many of the important manufacturing areas in Scotland. On the cliffs near Fidra we passed the remains of an observation post which would have had very good views of the estuary.
The search light stations along with a magnetic detection band of steel across the sea bed that lay from near Gin head to Isle of May then across to Fife which would alert any metal magnetic movement of submarines or ships trying to enter the Forth to attack the Navy Fleet  at Roysth,  If activity was detected fast patrol boats based on Isle of May and fighters at RAF Drem would be scrambled along with Naval frigates at Rosyth, The Forth was one of the most defended estuaries on the east coast. The defences here at Fidra and across the Forth Estuary in Fife at Kincraig were the outer line of defences.
(Extra) The nearby view looking east to the islands of  Fidra, The Lamb, Craigleith and Bass Rock.