Castlederg (earlier Caslanadergy, from Irish: Caisleán na Deirge, meaning "castle on the Derg") is a village in County Tyrone, Northern Ireland. It lies on the River Derg and is near the border with County Donegal. It stands in the townlands of Castlesessagh and Churchtown, in the historic barony of Omagh West and the civil parish of Urney. The village has a ruined castle and two ancient tombs known as the Druid's Altar and Todd's Den.