Castle (Castles, Cassels, Cassells), Richard (c.1690/95–1751), was one of at least four sons in a huguenot family and was possibly born in Kassel, Hesse, Germany; his parents' names are not known. An officer (c.1715) in a regiment of engineers, he travelled through Europe studying fortifications and canals before arriving (c.1725) in England, where he was probably acquainted with Lord Burlington's circle.
He is said to have been brought to Ireland by Sir Gustavus Hume to rebuild Castle Hume, Co. Fermanagh (1728–9), and Edward Lovett Pearce (qv) employed him as a draughtsman on Parliament House and regarded him highly. In 1730 Pearce was appointed surveyor general of the Newry canal, and Castle, his assistant, wrote an account of continental canal engineering, ‘An essay on artificial navigation’ (c.1729–30) (NLI, MS 2737). On Pearce's death (1733) Castle was appointed engineer to the Newry canal. In this capacity he supervised the first summit-level canal in Britain or Ireland and probably built the first stone lock chamber in Ireland (1734). In 1735 he was consulted over Dublin's water supply and published An essay towards supplying the city of Dublin with water (1735). For reasons unknown he was dismissed in 1736.
Having succeeded to Pearce's private practice, he became the most prolific and prominent architect in Ireland of his day. Considered a talented rather than a great architect, Castle consolidated renaissance architecture in Ireland, particularly the Palladian style. He contributed significantly to the development of Dublin, designing the first imposing town houses in cut stone for the nobility, notably Tyrone House, Marlborough St. (1740–45), built for Marcus Beresford (1694–1793), later earl of Tyrone, and Leinster House, Kildare St. (1745–51), for James Fitzgerald (qv), earl of Kildare, the grandest town house and since the 1920s the seat of Dáil Éireann. His commissions included 85 Stephen's Green (c.1738), the first stone-fronted house on the Green, latterly part of Newman House; houses in Kildare St., notably Doneraile House (designed c.1743); and Sackville Place.
His public works include buildings in TCD: the Printing House with its doric temple front (begun 1734), the dining hall (begun 1745, replaced 1765), and the belltower (begun c.1740), which dominated the front square; considered too insecure for the bell to be tolled, it was demolished in the 1790s. He also presented (c.1740) plans – now lost – for a new square and front to the college. He designed the Music Hall, Fishamble St. (opened 1740), which Handel praised for its acoustic properties after the premiere of ‘The Messiah’ (1742), and the (Rotunda) Lying-in Hospital (1750–57). Criticised for resembling a nobleman's palace rather than a hospital, it was completed after Castle's death by his clerk, John Ensor (qv).
Castle designed many country houses, including Belvedere, Co. Westmeath (designed 1740), which incorporated the ‘Venetian’ window, a common feature of his designs, and Ballyhaise, Co. Cavan (c.1733). By altering and enlarging many houses, he created grand country mansions (often with vaulted stables), notably Powerscourt, Co. Wicklow, with its magnificent Egyptian hall (built 1731×1740; damaged by fire 1974, and since partly restored), Westport House, Co. Mayo (1731–40), and Carton House, Co. Kildare (c.1739–45). Conolly's Folly at Castletown estate, Co. Kildare (1740), a tall obelisk mounted on multiple arches, is attributed to him. He possibly collaborated with Francis Bindon (qv) on Belan House, Co. Kildare, complete with temple and three obelisks (1743), and Russborough, Co. Wicklow (c.1742–55); built in Wicklow granite with its long facade composed of a centre block, colonnades, and flanking wings, Russborough is considered one of the finest examples of the Palladian style. In 1745 he was elected a member of the Dublin Society.
He always gave clear instructions to workmen. When he inspected his works, he required the attendance of all his workmen, who followed him in a long train; if dissatisfied, he frequently had the works pulled down. A man of great integrity, he was respected by the nobility not only as an architect but also as an agreeable companion. Though wealthy, he was improvident, drank heavily, and frequently accompanied Dr Bartholomew Mosse (qv) to a tavern, staying till the early hours of the morning. Somewhat eccentric, he had an aversion to shaving. He suffered from gout and died suddenly 19 February 1751 at Carton, while writing a letter; he was buried at Maynooth, Co. Kildare. Most of his surviving drawings are held in the IAA. He married (1733) Jane Truffet or Truphet of Lisburn, Co. Down, in the Huguenot Church, Dublin; there is no record of any children.