The heart of the Piazza del Duomo is, obviously, the Duomo, the medieval cathedral, entitled to Santa Maria Assunta (St. Mary of the Assumption). This is a five-naved cathedral with a three-naved transept. The church is known also as the Primatial, the archbishop of Pisa being a Primate since 1092.
Construction was begun in 1064 by the architect Busketo, and set the model for the distinctive Pisan Romanesque style of architecture. The mosaics of the interior, as well as the pointed arches, show a strong Byzantine influence.
The façade, of grey marble and white stone set with discs of coloured marble, was built by a master named Rainaldo, as indicated by an inscription above the middle door: Rainaldus prudens operator.
The massive bronze main doors were made in the workshops of Giambologna, replacing the original doors destroyed in a fire in 1595. The central door was in bronze and made around 1180 by Bonanno Pisano, while the other two were probably in wood. However worshippers never used the façade doors to enter, instead entering by way of the Porta di San Ranieri (St. Ranieri's Door), in front of the Leaning Tower, made in around 1180 by Bonanno Pisano.
Above the doors there are four rows of open galleries with, on top, statues of Madonna with Child and, on the corners, the Four evangelists.
Also in the façade we can find the tomb of Busketo (on the left side) and an inscription about the foundation of the Cathedral and the victorious battle against Saracens.
At the east end of the exterior, high on a column rising from the gable is a modern replica of the Pisa Griffin, the largest Islamic metal sculpture known, the original of which was placed there probably in the 11th or 12th century, and is now in the Cathedral Museum.
The interior is faced with black and white marble and has a gilded ceiling and a frescoed dome. It was largely redecorated after a fire in 1595, which destroyed most of the medieval art works.
Fortunately, the impressive mosaic, in the apse, of Christ in Majesty, flanked by the Blessed Virgin and St. John the Evangelist, survived the fire. It evokes the mosaics in the church of Monreale, Sicily. Although it is said that the mosaic was done by Cimabue, only the head of St. John was done by the artist in 1302 and was his last work, since he died in Pisa in the same year. The cupola, at the intersection of the nave and the transept, was decorated by Riminaldi showing the ascension of the Blessed Virgin.
Galileo is believed to have formulated his theory about the movement of a pendulum by watching the swinging of the incense lamp (not the present one) hanging from the ceiling of the nave. That lamp, smaller and simpler than the present one, it is now kept in the Camposanto, in the Aulla chapel.
The impressive granite Corinthian columns between the nave and the aisle came originally from the mosque of Palermo, captured by the Pisans in 1063.
The coffer ceiling of the nave was replaced after the fire of 1595. The present gold-decorated ceiling carries the coat of arms of the Medici.
The elaborately carved pulpit (1302-1310), which also survived the fire, was made by Giovanni Pisano and is one the masterworks of medieval sculpture. It was packed away during the redecoration and was not rediscovered and re-erected until 1926. The pulpit is supported by plain columns (two of which mounted on lions sculptures) on one side and by caryatids and a telamon on the other: the latter represent St. Michael, the Evangelists, the four cardinal virtues flanking the Church, and a bold, naturalistic depiction of a naked Hercules. A central plinth with the liberal arts supports the four theological virtues.
The present day reconstruction of the pulpit is not the correct one. Now it lies not in the same original position, that was nearer the main altar, and the disposition of the columns and the panels are not the original ones. Also the original stairs (maybe in marble) were lost.
The upper part has nine panels dramatic showing scenes from the New Testament, carved in white marble with a chiaroscuro effect and separated by figures of prophets: Annunciation, Massacre of the Innocents, Nativity, Adoration of the Magi, Flight into Egypt, Crucifixion, and two panels of the Last Judgement.
The church also contains the bones of St Ranieri, Pisa's patron saint, and the tomb of the Holy Roman Emperor Henry VII, carved by Tino da Camaino in 1315. That tomb, originally in the apse just behind the main altar, was disassembled and changed position many times during the years for political reasons. At last the sarcophagus is still in the Cathedral, but some of the statues were put in the Camposanto or in the top of the façade of the church. The original statues now are in the Museum of the Opera del Duomo.
Pope Gregory VIII was also buried in the cathedral. The fire in 1595 destroyed his tomb.
The Cathedral has a prominent role in determining the beginning of the Pisan New Year. Between the tenth century and 1749, when the Tuscan calendar was reformed, Pisa used its own calendar, in which the first day of the year on March 25, which is the day of the Annunciation of the Virgin Mary. The Pisan New Year begins 9 months before the ordinary one. The exact moment is determined by a ray of sun that, through a window on the left side, hit a shelf egg-shaped on the right side, just above the pulpit by Giovanni Pisano. This occurs at noon.
In the Cathedral also can be found some relics brought during the Crusades: the remains of three Saints (Abibo, Gamaliel and Nicodemus) and a vase that it is said to be one of the jars of Cana.
The building, as have several in Pisa, has tilted slightly since its construction.