Presented as a secret facility located in secured waters
somewhere off the British Islands, the maximum security prison
facility code-named 'Sherrinford' is actually a Napoleonic fort
located just offshore from the town of Tenby in Wales.
The island itself becomes connected to Tenby itself only at low
tide, making it an ideal defensive position for the area
encompassing Milford Haven and what would become Pembroke Dock. The
fort was constructed between 1867 and 1870 to house a coastal
defence battery of nine guns and a garrison of one hundred men. The
fort was used on and off as a manned military position during the
First and Second World Wars, though before that the structure was
bought in 1907 for private ownership. Between 1968 and 1979
the site played host to a zoo before the business relocated, and
has since stood empty.The fort reopened to the public in May 2015
for tours, but closed again in August 2016, just after filming on
The Final Problem wrapped.
Scenes of helicopter landings to access the Sherrinford facility
were filmed at Southerndown Beach in Porthcawl - most recognisable
as Bad Wolf Bay in several memorable episodes of David Tennant-era
Doctor Who.
The extensive interiors for the Sherrinford facility were
designed and constructed by production designer Arwel Wyn Jones and
his team on sound stages at Pinewood Studios Wales.
Scenes on the fishing boat that Sherlock, John (and presumably
Mycroft) conceal themselves on to approach the island were filmed
safely moored in Cardiff Bay, on a tuna fishing boat named the
Tunnara II. The small boat is a former lifeboat - of the Watson
class.