When the Civil Survey was conducted between 1654 and 1656, it recorded that William Fleming, Lord of Slane, owned most of the parish in 1640. The surveyors noted that his manor featured ‘a castle with a hall, a stable Brewhouse, and Coach house, and divers other houses of Office’, describing it as the mansion house of the Lord of Slane himself.

A 17th century parish map of Slane offers a tantalising glimpse of the original Fleming castle, which stood where the current building now rises. According to historian Westropp’s 1901 interpretation of this map, the castle formed a large courtyard complex with an impressive array of defensive features. The river front boasted a strong square tower with large windows at the south east corner, a round tower with a pointed roof to the west, and various buildings including another high square tower at the south west. The landward sides were protected by battlemented walls, a gatehouse with its own large archway on the north face, and additional round towers at the corners, all surrounded by a defensive fosse or ditch. Whilst this description might represent some artistic licence in interpreting a simplified drawing, it paints a picture of a substantial fortified residence.

The castle that visitors see today bears little resemblance to its medieval predecessor, having been completely reimagined in 1785 to a design by James Wyatt. This seven bay, three storey Gothic Revival castle rises above a basement level, with square corner towers that may incorporate fragments of earlier stonework. Its most striking feature on the south west front is a large bow window that houses the castle’s famous circular ballroom, a testament to the Georgian era’s love of dramatic architectural statements. Despite this radical transformation, the site continues to command the same strategic position above the Boyne that made it such an important seat of power for centuries.