Craig y Cilau is a limestone escarpment in the Brecon Beacons National Park in Powys, Wales. It was declared a national nature reserve in 1959 because of its importance for wildlife, particularly plants. It is situated on the north side of Mynydd Llangatwg in Llangattock community, about 2 miles (3.2 km) south-west of Crickhowell. About 50 species of bird breed on the reserve including ring ouzel and raven on the cliffs and redstart, wood warbler and pied flycatcher in the woods. One of the caves, Agen Allwedd, holds a roost of lesser horseshoe bats during the winter months.