Castell Mawr (known in English as Castle Rock) is a large limestone rock which dominates the western edge of Anglesey, north Wales. Overlooking Red Wharf Bay, it was probably once the site of an Iron Age fort but today the rock is a protected refuge for nesting seabirds. It is located between the village of Red Wharf Bay and the promontory where St. David's Holiday Park is sited. The closest villages are Pentraeth and Benllech.