Llyn Cwellyn is a reservoir which stands on the Afon Gwyrfai in Nant y Betws, between the Snowdon Massif and Mynydd Mawr, in the northen side of Wales, in Gwynedd region. Its maximum depth reaches 37 m (120 feet) and has a surface of 87 ha( 215 acres). It is very deep and is one of the few lakes to support a natural population of Arctic char. It is also the home to brown trout and otters can be observed, too. At the northern side of the lake is situated Castell Cidwm, known also as Wolf Castle.