The basilica, which was begun in 1228, is built into the side of a hill and comprises two churches (known as the Upper Church and the Lower Church) and a crypt, where the remains of the saint are interred. The interior of the Upper Church is an important early example of the Gothic style in Italy. The Upper and Lower Churches are decorated with frescoes by numerous late medieval painters from the Roman and Tuscan schools, and include works by Cimabue, Giotto, Simone Martini, Pietro Lorenzetti and possibly Pietro Cavallini. The range and quality of the works give the basilica a unique importance in demonstrating the outstanding development of Italian art of this period, especially if compared with the rest of Christian Europe.