Carlton House, 1814

In the spring of 1814, with Napoleon defeated, the Prince Regent ordered temporary rooms for the gardens of Carlton House. At their centre was Nash's 24-sided ballroom, the Polygon Room. It went up in about ten weeks: Nash's chief carpenter, William Nixon, built it under his directions, Jeffry Wyatt's firm did the carpentry, and John William Hiort, Clerk of Works and Measurer, ran the job. That July the Chancellor of the Exchequer told the Commons it could be taken down and put up again at pleasure. The House laughed. Four years later it was.

Its first night was the fête for the Duke of Wellington on 21 July 1814, a reception for between 1,800 and 2,000 guests that went on until dawn. On 1 August it stood, still furnished, behind the garden wall while London celebrated the Grand National Jubilee in the parks. The Prince kept it as his summer ballroom until 1818.

The structure

The roof is a conoid, tent-like rather than domed, carried by 24 principal rafters rising from the perimeter to a central ring at the apex. The ring is formed by the rafters' own upright members, bevelled and bound together with iron hoops like the staves of a barrel. The rafters are laminated from planks fastened with iron straps, bolts and pegs, decades before laminated timber became an industrial product.

The outward thrust at each rafter foot is resolved by cross-bracing between two rings of 24 columns, so the roof stands on its own and passes only vertical load below. The room had no central column: a tent without a pole. Writing in 1830, after Nixon's death, T. F. Hunt credited the roof to him: it "was designed or invented by, and executed under the direction of, the late William Nixon." Nixon later worked at the Brighton Pavilion, whose tented roofs use the same method.

Transfer to Woolwich, 1818–1820

In August 1818 the Prince Regent ordered the building moved to Woolwich for the Royal Military Repository. Nash wrote the method: strike off the iron hoops, lower each rafter whole and number every part, because the building "has been constructed so as to take to pieces and be refixed." Five men took it down that autumn, with Nixon on site to explain it and William Wyatt numbering the parts.

The Repository had been founded in 1778 by Captain William Congreve the elder as a school for training artillerymen. After his death in 1814 his son, Sir William Congreve, 2nd Baronet, secured the building for Woolwich.

At Woolwich the timber walls were replaced by a brick drum, and in 1819 a central Doric column was added, the structural solution chosen for a building that was now permanent. Nixon's cross-bracing between the column rings was retained. The building was declared fit to occupy in May 1820.

The Royal Artillery Museum, 1820–2001

The Rotunda opened as a museum in 1820, one of the world’s earliest artillery museums. The younger Congreve set out its aims as providing "practical instruction for the Officers, Non-commissioned Officers, and Men of the Regiment of Artillery, both on the great scale and with the aid of Drawings and Models." The collection grew to include ordnance, models, trophies and instruments, among them rockets captured from Tipu Sultan's army at Seringapatam in 1799.

The central Doric column bears the painted names of 39 Royal Artillery officers, from Colonel Albert Borgard, the regiment's first Colonel from 1722, to General Sir Edward Bruce Hamley in the 1890s. They include commanders, scientists, explorers and inventors: Colonel Sir Augustus Simon Frazer, who led the Royal Horse Artillery at Waterloo; General Sir Edward Sabine, President of the Royal Society; Captain William Congreve the elder; and two holders of the Victoria Cross.

In the 1860s, under General Sir John Henry Lefroy, the canvas was replaced with lead, a far heavier load on the timber. The lead is now itself historic fabric.

A major restoration was carried out between 1972 and 1975. The trusses were found sound and repaired in place, which is why Nash's rafters survive. Elsewhere the intervention was heavy: the outer cast iron columns were encased in concrete, the inner columns clad in timber, the lead relaid and a concrete slab laid throughout.

The museum closed in 2001, when the collection moved to Firepower at the Royal Arsenal.

Decline, 2001 to the present

The building kept part of the reserve collection until around 2010 and has had no sustained use since. It was placed on Historic England's Heritage at Risk Register in 2007, classified as Priority A: immediate risk of further rapid deterioration or loss of fabric, no solution agreed.

In autumn 2023 emergency stabilisation was carried out under Crosby Granger Architects. The 1974 canvas ceiling was taken down for inspection, and eight defective trusses were propped, with a ninth in winter 2024–25. The condition survey, updated in January 2025, put repairs at about £3 million.

The disposal

The Rotunda, on Metropolitan Open Land, is being sold by the Defence Infrastructure Organisation. Rotunda Trust has been formed to acquire it and secure its long-term conservation. Read about the trust.