Cefn yr Ogof is the highest point of the hill to the west of Gwrych Castle near Abergele and Llanddulas. It is 204 m (669 ft) high. The view from the top is one of the hidden delights of the North Wales coast with views towards the Clwydian Hills, Denbigh Moors, (Mynydd Hiraethog) Snowdonia, the Great Orme, Anglesey and the Dulas valley. The name refers to the caves located on the hill.

It has recently been promote to HuMP status, having a prominence of over 100 metres.

Link to Cefn y ogof, hosted by the Gwrych Castle website.   Llanddulas Limestone and Gwrych Castle Wood is an SSSI including the hill.

• Riellaceae
• Rielle Carrington
• Riells i Viabrea
• Riel Nason
• Riel (provincial electoral district)
• Riemann curvature
• Riemann fractional integral
• Riemann functional equation
• Riemannian Geometry
• Riemannian integral
• Riemannian Manifold
• Riemannian theory
• Riemann invariant
• Riemann mapping
• Riemann moduli space
• Riemann relations
• Riemann roch
• Riemann Roch for curves
• Riemann-Roch theorem for algebraic curves
• Riemann-Roch theorem for Riemann surfaces
• Riemann's functional equation
• Riemann-Silberstein vector
• Riemann's integral
• Riemann's Minimal Surface
• Riemann's minimal surface