as a kid i am sure parents took me there or that as old man worked for large civil engineers that undertook work on such structures etc ( one of them being the welsh devils bridge repair work )
You could spend a good day in this area there is plenty to see and do
The Dam is high up in the mountain next to the Crimea Pass a lovely piece of road the whole of that area was a slate mine and everything is gray, known locally as the @rsehole of Wales, but there is much more to it. Llechwedd Slate Caverns is well worth a visit and is a real eye opener to the conditions that the old minors had to endure. You can go deep into the mine to see how the granite rock was extracted and brought up to the surface and at the end of the process made into roofing tiles. From memory I think the Mine has 14 levels and over 50 miles of underground railway tracks.
I don’t normally go this way when we take a group up as it can take the whole day up. It may be worth a trip up to the dam.