The way rooms have been depicted in S3 and S4 emphasis the often dreamlike quality of the episodes. Here are some examples.
The camera is moving away from Sherlock while he is destroying the coffin, moving through the chimney, the grille, zooming away from the angry man, higher and higher into a bird’s-eye view of the fortress and the whole island.
Later we get a variation of this perspective, showing us Sherlock from above in a very similar room that is furnished with a very similar grille and very similar chimney, leaving in air and moonlight.
Shortly after, the room is revealed as fake, up to the chimney with the grille.
Why show us a fake room that is so very similar to the “real” one in Sherrinford?Why re-create a Sherrinford room in a container in front of Sherlock’s childhood home? Why reconstruct the room at all if not to insinuate that we - just as Sherlock - cannot trust our eyes or minds?
But there is more. Here you see three types of walls which are all similar: the padded cell from HLV, Eurus’s cell, and the well in which John is trapped. Walls made of vertical slabs, leaving in light through slits or from above (moonlight in the well).
And as if this was not enough, there is this (please note the similar colour schemes):
Both are cells in which to keep patients who are mentally ill - one Victorian-style padded cell with a chained Moriarty (as befits a someone who loves to create Victorian scenarios in his head), the other one a cool, high-tech version of a cell (where its inmate also meets a cool, styled unchained Moriarty). I am quite convinced that both rooms only exist in Sherlock’s head (this is why their looks and function are so similar). As does the well. And everything we see in TFP.
Bird’s eye view of the junction of US Route 101 and US Alternate 101 at Capistrano Beach, California. The cars appear to be 1940s-50s but they’re too small to be sure. The railroad overpass advertises the Santa Fe Railroad. No other date or source info available.