The school building incorporates the remains of what was once Clongowes Wood Castle, a 15th century tower house that came into the possession of Anne Eustace in 1447. The original castle met a violent end when it was blown up during the turbulent events of 1641, but its legacy lived on in ways that weren’t fully understood until archaeological excavations in the early 2000s revealed the true extent of the historic structures concealed within the modern school.

The investigations, carried out between 2002 and 2003, uncovered evidence of a 17th century fortified house built on the site, which itself may have incorporated fabric from Anne Eustace’s earlier tower house. The fortified house was an impressive T-shaped structure with flanking towers at either end of its main façade, protected by an external defensive fosse. The substantial thickness of some internal walls suggests that builders of the later house pragmatically reused portions of the medieval tower house, creating a palimpsest of Irish defensive architecture spanning multiple centuries.

Over time, the building continued to evolve with the changing tastes and needs of its occupants. The 18th century saw the addition of elegant neo-classical interiors that transformed the once purely defensive structure into something more refined, whilst a Gothic Revival extension was added to the rear in later years. Today, these layers of history; from medieval tower house to 17th century fortification to Georgian elegance and Victorian Gothic; all coexist within the fabric of the school, making it a remarkable example of architectural continuity and adaptation through five centuries of Irish history.