Castlederg Castle, or Derg

Derg Castle, also known as Castlederg Castle, is a post-medieval ruined fortification located on the north bank of the River Derg within the town of Castlederg in County Tyrone, Northern Ireland. The town itself derives its name from the Irish Caisleán na Deirge, meaning Castle on the Derg. While the earliest recorded structure at this strategic river ford dates to 1497 as an O'Donnell tower house, the current ruins primarily reflect early 17th-century modifications. Originally a site of contention between the warring O'Neill and O'Donnell clans, the property was granted to Sir John Davies in 1610 under the Plantation of Ulster. Davies reconstructed the site into a substantial stone and lime bawn featuring three open flankers, intending it to serve as a key defensive stronghold connected by a long causeway to his nearby Kirlish Castle.

The castle holds significant historical importance for its role during periods of intense regional conflict. In 1641, it served as a vital refuge for the local Scottish settler community, successfully withstanding a siege by forces led by Sir Phelim O'Neill until ammunition supplies were exhausted. Decades later, during the Williamite War in 1689, the garrison surrendered to King James II's troops after receiving promises of safe passage, marking the final military use of the structure. Following these events, the castle was abandoned and gradually fell into ruin. Today, the site is designated as a Scheduled Historic Monument administered by the Northern Ireland Department for Communities. Although unfinished even by 1622 standards, its remaining earthworks and foundations stand as a testament to the turbulent plantation era and the strategic military architecture of County Tyrone.