My parents taught me that this was the remains of Denbigh Mental Hospital; it's only now, verifying supplementary information, that I've discovered the truth.

'Gwylfa Hiraethog', a hunting lodge, was built in about 1908 for Hudson E. Kearley MP, later ennobled as Viscount Devonport. It was said to be the highest inhabited location in Wales, at 496 m asl; the nearby 'Sportsman's Arms' claims to be Wales' highest pub.
As cars became more common, the Denbigh Moors ceased to be so remote as they had once been, and the need to maintain and staff a household for visitors to stay overnight diminished. The lodge fell into disuse in the 1950s, gradually becoming skeletal, and hence distinctive as a landmark. I remember it being more substantial in the 1980s (I particularly recall spiky chimneys), so there must have been severe collapses since I was here last.

Incidentally, the mental hospital is in Denbigh, and intact.