Visconti had ambitions to follow the newest trends in European architecture. In 1389, a French chief engineer, Nicolas de Bonaventure, was appointed, adding to the church its Rayonnant Gothic, a French style not typical for Italy. He decided that the brick structure should be panelled with marble. Galeazzo gave the Fabbrica del Duomo exclusive use of the marble from the Candoglia quarry and exempted it from taxes. Ten years later another French architect, Jean Mignot, was called from Paris to judge and improve upon the work done, as the masons needed new technical aid to lift stones to an unprecedented height. Mignot declared all the work done up till then as in pericolo di ruina ("peril of ruin"), as it had been done sine scienzia ("without science"). In the following years Mignot's forecasts proved untrue, but anyway they spurred Galeazzo's engineers to improve their instruments and techniques. Work proceeded quickly, and at the death of Gian Galeazzo in 1402, almost half the cathedral was complete. Construction, however, stalled almost totally until 1480, for lack of money and ideas: the most notable works of this period were the tombs of Marco Carelli and Pope Martin V (1424) and the windows of the apse (1470s), of which those extant portray St. John the Evangelist, by Cristoforo de' Mottis, and Saint Eligius and San John of Damascus, both by Niccolò da Varallo. In 1452, under Francesco Sforza, the nave and the aisles were completed up to the sixth bay.