When Worcester was appointed to the post of President of the Council of Wales and the Marches, he decided that he needed a seat in the Marches appropriate to his status. The house was primarily intended to be used for ceremonial and official, rather than domestic, purposes. The architect is unknown. The house was built of mottled pink and grey blocks of Old Red Sandstone, probably mostly reused from the gatehouse and Great Tower of the castle, which were wholly or partly demolished after the Civil War; much has been renewed more recently. The remains of the castle stand in the far left-hand side of the courtyard in front of the house. The facade of the house is severely symmetrical with an elegant and decorative doorway. The two side wings are 19th-century additions to a building that originally stood alone. The interior is described as "astonishing", particularly for its plasterwork. On the first floor there is a large room with extravagantly decorated ceilings; this was originally five rooms, later combined as the main courtroom. The plasterwork was mostly made by travelling craftsmen using established patterns, but has been lavishly and ostentatiously fashioned with pendant floral festoons laced with ribbons in what were originally the Marquis's private rooms. Shortly after building Great Castle House, Worcester was created Duke of Beaufort, and needed still larger houses, first at Troy House and then at Badminton House, leaving Great Castle House to be repanelled for the Assizes. When the Assizes moved to Shire Hall in 1725, the house became lodgings for the judges, but later they moved to less spacious lodgings in the town, and from about 1760 the house provided a high-class school for young ladies.
In 1853, it became the headquarters of the Militia Regiment, later the Royal Monmouthshire Royal Engineers, the Senior Regiment of the Reserve Army. Most of the building remains as the regiment's headquarters, but part now houses the Castle and Regimental Museum, established in 1989, which is open in the afternoon in summer. The Museum opens onto the King's Garden, containing plants known to have been growing at the time of Henry V, who was born in the castle. The museum displays include regimental artefacts and regalia, the importance of the Dukes of Beaufort, the earlier defences of Monmouth and its castle, HMS Monmouth, and the military activities of the regiment from its founding to the present.
