Llansteffan Castle, Carmarthenshire, Wales, Llansteffan Castle is a privately owned castle in Llansteffan, Carmarthenshire, Wales, overlooking the River Tywi estuary in Carmarthen Bay. The castle was built by the Normans after 1100 as part of their invasion of Wales. The castle was captured twice by the forces of Owain Glyndŵr in 1403 and c.1405. The castle is under a deed of guardianship with CADW.