From 1557 he worked almost exclusively for Cosimo I de' Medici, Lanci's chief employer and patron for whom he supervised such projects as the fortification of Siena and many other Tuscan towns. His urban works included the design of Terra del Sole a fortified new city for Cosimo I de' Medici, in what is now the Province of Forlì-Cesena. Lanci also designed the church of Santa Maria della Rosa in the spa town of Chianciano Terme, in today's Province of Siena; Santa Maria della Rosa was built on a central plan; even though it was not finished, the church is notable for the sobriety, and clean lines of its design which culminates in an elegant dome. The church, however, was never fully completed. In 1585 building work stopped, fourteen years after Lanci's death. He also designed the walls of Grosseto in 1564, directing the construction site until his death in 1571, when he was succeeded by his son Marino.