Mahee Castle, also known as Nendrum Castle, is a small ruined tower house located on Mahee Island in County Down, Northern Ireland. Built in 1570 by Captain Thomas Browne, this historic site overlooks Strangford Lough and was abandoned by the early 17th century. Despite its current state of disrepair, efforts have been made to partially renovate the tower house and preserve its historical significance. The castle's strategic location near the border of two territories and its role in border conflicts during the 15th and 16th centuries add to its intriguing history.