By the beginning of the fifteenth century, after a hundred years of construction, the structure was still missing its dome. The basic features of the dome had been designed by Arnolfo di Cambio in 1296. His brick model, 4.6 metres (15 ft) high 9.2 metres (30 ft) long, was standing in a side aisle of the unfinished building, and had long ago become sacrosanct. It called for an octagonal dome higher and wider than any that had ever been built, with no external buttresses to keep it from spreading and falling under its own weight.