It says on the top line of the name plate on the structure Ffynnon then Riddw ? According to what I can find Ffynnon translates to Well or Fountain. looking at it Well seems more likely. Maybe the second line clarifies it but I can't really make out what it says.
Much in Wales depends on the local environment - north or south, coal or iron?
Staggered doorways indicate explosives protection, as does the lack of any clear sign of a solid roof (explosives buildings roofs are often light, so that they blow straight off allowing the blast ro go upwards) great number of old munitions workings in Wales. Tjose walls look pretty solid.
I am half Welsh, and lived in Wales for several years as a child. I know this spot, as my father used to love the view from the road, and took the family there for picnics in the summer.
The inscription reads Ffynnon Eidda, which translates as Ffynnon's Well. It's an old (very old) spring-fed cattle drinking well. It's on the B4407, a road that runs across the high moors between Ysbyty Ifan and Ffestiniog.
During the 1700s and 1800s big herds of cattle were driven along the road on their way to market, and this well was a famous watering spot.
The inscription below the name reads Yf a bu ddiolchgar, which means 'Drink, and be thankful'.