The building is preceded by a large courtyard terrace to have served as a cemetery. He had been brought in specially made ??with earth from Jerusalem, whether for religious reasons, both for its special composition, which gave the characteristic of rapidly mummify the bodies that were buried.
The facade is flanked by two massive towers, lightened by elegant mullioned windows and surmounted by pyramidal spiers added in the fifteenth and different from each other: a square plan and battlements shaped flames, which would symbolize the papal miter and the power of the Church, while the other, an octagonal plan and Ghibelline battlements, the royal crown and the temporal power. The porch before the fifteenth century façade, with three strings (two pointed and one rounded arch) supported by four columns, and with ribbed vaults. Under the porch is the "Gateway Regum", embellished with a marble portal finely decorated, with paintings on the sides.