 On November 15, 1290, Pope Nicholas IV laid the cornerstone for the present building and dedicated it to the assumption of the Virgin, a feast for which the city had a long history of special devotion. The design has often been attributed to Arnolfo de Cambio, but the prevailing modern opinion is that the master mason was an obscure monk named Fra Bevegnate from Perugia. The church is striped in white travertine and greenish-black basalt and narrow bands, similar in many ways to the Cathedral of Siena and other central Italian cathedrals of that era. In the following decade, cathedral authorities called Sienes architect and sculptor Lorenzo Mitanni to stabilize the building and design of façade. He enlarged the choir and planned a transept with two chapels, spaces that were not finished until long after his death. The cathedral has five bells, tuned in E-flat, which date back to the Renaissance. The façade is particularly striking and includes some remarkable sculpture by Lorenzo Mitanni. Inside the cathedral, the Chapel of San Brizio is frescoed by Francis Angelico, and with Luca Signorelli's masterpiece, his last judgment. The corporal of Bolsena, on view in the Duomo, dates from a Eucharistic miracle in Bolsena in 1263, when a consecrated host began to bleed onto a corporal, the small cloth upon which the host and chalice rest during the canon of the mass. The city of Orvieto has long kept the secret of its labyrinth of caves and tunnels that lie beneath the surface. Dug deep into the tough, a volcanic rock, these secret hidden tunnels are now open to view only through guided tours. Their spectacular nature has also yielded many historical and archaeological finds. St. Anselm College has set up a program where each summer, students travel to Italy to work at the college's archaeology site located at the Corillia excavation site, just outside town. The underground city boasts more than 1,200 tunnels, galleries, wells, stairs, quarries, cellars, unexpected passageways, cisterns, superimposed rooms with numerous small square niches for pigeon roosts, detailing its creation over the centuries. Many of the homes of noble families were equipped with a means of escape from the elevated city during times of siege through secret escape tunnels carved from the soft rock. The tunnels would lead from the city Palazzo to emerge at a safe exit point some distance away from city walls. Work on the construction of the Palazzo del Capitano del Popolo began in the 13th century on an area that had been occupied since 1157 by the papal palace built under the reign of Pope Hadrian IV. The original Palazzo del Capitano was a single ground floor logia that was used as a market place or for meetings, from which the magistrate would speak to the citizens. This was where the surrounding lords or representatives of vanquished cities came to pay their allegiance to Orvieto. The structure was enlarged within 10 years of its original construction and, in 1315, the bell tower was added and in the subsequent year, a great bell was hung there. The upper part of the structure was covered in 1472 and the large hall divided into two rooms, one large and the other small. The larger of the two occupied an area that corresponds approximately to the room known today is the Salade Quadricento. Subsequently, the building functioned as a residence for the Capitano del Popolo, the Podesta and the Signori set. From 1596 one of the lower section rooms housed the studio, which had been reinstituted a few years earlier by Lorenzo Magalotti. Students of law, theology and logic came here to study twice a day, each time the bell of Palazzo del Popolo rang, until 1651. Few records of this university appear after this date. Some sources indicate that it dates back to 1113 and had connections with names such as the Benedictine monks Graziano and Gozio of Orvieto. In Piazzacan stands the Forteza del Alborno. It was built by order of the Spanish Cardinal Alborno under orders from Pope Innocent VI and designed by Condodiero and military engineer Ugalino de Montemarte. The Alborno's fortress stands on an area that was once occupied by a temple known by the Latin name Agaril. Originally known as the Rocca di San Martino, construction on this massive fortress started either in 1359 or 1353 near the town cemetery. Its aim was to provide the church a secure site in the city and allow the Cardinal and his captains to consolidate recent military victories. In its original square plan the fortress was flanked by a small building near the main entrance and surrounded by a moat, crossed by a drawbridge. However, the Rocca was almost completely razed to the ground in 1395 and successive attempts to rebuild it were unsuccessful. The fortress was finally rebuilt during the mid-15th century, using original plans and an additional circular line of fortifications. After the sack of Rome at the end of 1527 Pope Clement VII took refuge in Orvieto. To ensure that the city would be sufficiently supplied with water in the event of a siege, he gave orders for the digging of the now famous artesian well Pozzo di San Patrizio. For added security, the Pope ordered that a second well be dug to supply the fortress alone.