Suomenlinna Sea Fortress, Helsinki, Finland

This huge set of fortifications is spread over five islands at the entrance to Helsinki harbour in the Baltic Sea.  Construction began in 1748, while the area was under Swedish control and after the Finnish War, the fortress was surrendered to Russia.  It remained under Russian control for over 100 years, being restored to Finland in 1918.  The fortress continued to be used into WW2 and was designated a UNESCO World Heritage Site in 1991.  The sheer scale of this site is incredible, there are examples of 19th Century and WW2 fortification everywhere, and many pieces of artillery still in situ.  Bridges link the individual islands and there are gun batteries, tunnels, casemates and magazines everywhere.  The main island hosts museums, cafés, shops, a brewery and accommodation for the 800 residents on the islands.  Some of the smaller islands have more hidden gun batteries, which can be freely explored and are in very original condition.  Although a very popular tourist attraction, and easily visited by regular ferries from Helsinki, it really didn't feel 'touristy' at all due to its size and the unrestored state of many areas.  This was a real positive as was the openness of areas, including unlit tunnels with uneven floors, which in most places would have been closed off to visitors!  Good footwear and a torch are recommended if you would like to explore these areas!