He was the leading artist from Siena in the second half of the 15th century. He produced many works in Siena; he was also a talented diplomat, leading to tenures at the courts of Naples, Milan and Urbino. During his own time, he was most noted as an architect, particularly for his work as a military architect, including his role in the development of the Bastion, which became the basis of post-medieval fortifications. Some of his major sculptures are 4 bronze angels for the high altar of Siena Cathedral. This widely traveled artist was friends with Leonardo; he wrote a treatise on architecture, that was not published until 1841. Italian sculptor and architect.