In February 1816 the first prisoners were admitted during the construction process, 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, in form of a raft: the first known use of this material for foundations in Britain since the Roman Empire. However, this added considerably to the construction costs, which eventually totalled £500,000, more than twice the original estimate and equivalent to £500 per cell.