Ardee Castle, Medieval tower house in Ardee, Ireland

Ardee Castle, also called St. Leger's Castle, is a rectangular tower house four storeys high, with turrets at the north-west and south-west angles. The main entrance was protected outside by a machicolation and inside by a murder-hole. The ground floor has a rounded barrel vault, and a stairway in the north-west turret reaches the upper storeys. A twin-light ogee-arched window survives at ground level.

John St. Leger built the tower house in the 15th century to defend the Pale. In 1641 Sir Phelim O'Neill seized Ardee and made the castle his army's headquarters, but English forces under Sir Henry Tichborne retook the town and installed a garrison. James II used it as his headquarters before the Battle of the Boyne in 1690.

Ardee Castle is known locally as St. Leger's Castle, after John St. Leger, who built it in the 15th century. Its Irish name is Caisleán Bhaile Átha Fhirdhia. The building is a National Monument and served as Ardee's district courthouse until June 2006, when a specialised facility replaced it.

Ardee Castle stands on Castle Street in the town of Ardee, County Louth, and is a National Monument. It served as the district courthouse until June 2006, when a specialised facility replaced it, so the interior is not presented as a visitor attraction with guided tours.

Ardee Castle is the largest fortified medieval tower house in Ireland or Britain. It is a National Monument, and a twin-light ogee-arched window survives at ground level, while three single looped, internally splayed windows remain on the fourth floor.