The Conolly Folly was designed by him, and built in 1740 as a park ornament for Castletown House. Russborough House (1742) Russborough was designed by Cassels for Joseph Leeson, 1st Earl of Milltown. It was built between 1741 and 1755. A central block containing the principal rooms is flanked by curved and segmented colonnades leading to two symmetrical service blocks. The main entrance, at the centre of one of Cassels's trademark 'suggested' porticoes, is on a raised piano nobile. It is reached by a broad flight of steps. The principal feature of the interiors are the rococo plaster-work and the ceilings, again by the master stuccoists Paul and Philip Lanfrachini; and ornate carved marble fireplaces, all contrasting with the austerity of the exterior. Summerhill House (1731) Summerhill House was a vast Palladian mansion in County Meath originally designed by Pearce, who died before the project was commenced. Cassels took over the project and was responsible for the rococo interiors. The building was damaged by fire in the 1920s and finally demolished in the 1970s. Powerscourt House (1741) Powerscourt House, Wicklow, was a large country house, originally a 13th-century castle, which was completely rebuilt by Cassels, starting in 1730 and finishing in 1741. The demesne was approximately . The three-story house had at least 68 rooms. The entrance hall was long and wide where family heirlooms were displayed. The main reception rooms were on the first floor rather than more typically on the ground floor. King George IV was the guest of Richard Wingfield, 5th Viscount Powerscourt in August 1821. Mervyn Wingfield, 7th Viscount Powerscourt inherited the title and the Powerscourt estate, which comprised of land in Ireland, at the age of 8 in 1844. When he reached the age of 21 he embarked on an extensive renovation of the house and created new gardens. Inspiration for the garden design followed visits by Powerscourt to ornamental gardens at the Palace of Versailles, Schönbrunn Palace, Vienna and Schwetzingen Palace near uHeidelberg. The garden development took 20 years to complete in 1880. On a commanding hilltop position, Cassels deviated slightly from his usual sombre style, to give the house something of what John Vanbrugh would have called the 'castle air' – a severe Palladian facade terminated by two circular domed towers. The house was destroyed by fire in 1974 when it was owned by the Slazenger family and renovated in 1996. In the