Gwynant, you're spot on so well done.
The Go Below office is part of the Conwy Falls Cafe and is on the left behind the building and that's why you probably haven't seen it. To be honest we didn't know about it until we stopped at the cafe for a coffee after having a walk up to Capel Garmon yesterday.
Just to try and explain the clues:-
Sir Clough Williams Ellis was given the job of designing the building
It's a two storey building and you can't
go below to see it, you must go up to the first floor level to see it on display.
The old Roman Road of Watling Street, now called the A5 is next to the building