This 15th-century cathedral was built by the architect Rossellino under the influence of Alberti. The travertine facade is divided in three parts, with Renaissance arches under the pope's coat of arms encircled by a wreath of fruit. Inside, the cathedral is simple but richly decorated with Sienese paintings. The building's perfection didn't last long—the first cracks appeared immediately after the building was completed, and its foundations have shifted slightly ever since as rain erodes the hillside behind. You can see this effect if you look closely at the base of the first column as you enter the church and compare it with the last.