Projects in which he used concrete foundations included the Millbank Penitentiary, the rebuilding of the London Custom House and the British Museum. At the first two he was called in when work overseen by previous architects had proved unstable. The prison at Millbank (1812–21; demolished c. 1890) had been designed by an architect called William Williams but his plan was then revised by Thomas Hardwick. The largest prison in Europe, it consisted of a hexagonal central courtyard with an elongated pentagonal courtyard on each outer wall of the central courtyard; the three outer corners of the pentagonal courtyards each had a tower one storey higher than the three floors of the rest of the building. Work had started under Hardwick in late 1812, but when the boundary wall it reached a height of about six feet high it began to tilt and crack. After 18 months, with £26,000 spent, Hardwick resigned. Work continued and by February 1816 the first prisoners were admitted, but the building creaked and several windows spontaneously shattered. Smirke and the engineer John Rennie the Elder were called in, and they recommended demolition of three of the towers and the underpinning of the entire building with concrete foundations: the first known use of this material for foundations in Britain since the Roman Empire. The work cost £70,000, bringing the total cost of the building to £458,000.