The heart of the Piazza del Duomo is 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 Buscheto, 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.
The Baptistery, dedicated to St. John the Baptist, stands opposite the west end of the Duomo. The round Romanesque building was begun in the mid 12th century: 1153 Mense August fundata fuit haec ("In the month of August 1153 was set up here..."). It was built in Romanesque style by an architect known as Diotisalvi ("God Save You"), who worked also in the church of the Holy Sepulchre in the city. His name is mentioned on a pillar inside, as Diotosalvi magister. the construction was not, however, finished until the 14th century, when the loggia, the top storey and the dome were added in Gothic style by Nicola Pisano and Giovanni Pisano.
It is the largest baptistery in Italy. Its circumference measures 107.25 m. Taking into account the statue of St. John the Baptist (attributed to Turino di Sano) on top of the dome, it is even a few centimetres higher than the Leaning Tower.
The portal, facing the façade of the cathedral, is flanked by two classical columns, while the inner jambs are executed in Byzantine style. The lintel is divided in two tiers. The lower one depicts several episodes in the life of St. John the Baptist, while the upper one shows Christ between the Madonna and St John the Baptist, flanked by angels and the evangelists.
The immensity of the interior is overwhelming, but it is surprisingly plain and lacks decoration. It has notable acoustics also.
The octagonal font at the centre dates from 1246 and was made by Guido Bigarelli da Como. The bronze sculpture of St. John the Baptist at the centre of the font, is a remarkable work by Italo Griselli.
The pulpit was sculpted between 1255-1260 by Nicola Pisano, father of Giovanni Pisano, the artist who produced the pulpit in the Duomo. The scenes on the pulpit, and especially the classical form of the naked Hercules, show at best Nicola Pisano's qualities as the most important precursor of Italian renaissance sculpture by reinstating antique representations. Therefore, surveys of the Italian Renaissance usually begin with the year 1260, the year that Nicola Pisano dated this pulpit.
The campanile (bell tower) is located behind the cathedral. The last of the three major buildings on the piazza to be built, construction of the bell tower began in 1173 and took place in three stages over the course of 177 years, with the bell-chamber only added in 1372. Five years after construction began, when the building had reached the third floor level, the weak subsoil and poor foundation led to the building sinking on its south side. The building was left for a century, which allowed the subsoil to stabilise itself and prevented the building from collapsing. In 1272, to adjust the lean of the building, when construction resumed, the upper floors were built with one side taller than the other. The seventh and final floor was added in 1319. By the time the building was completed, the lean was approximately 1 degree, or 2.5 feet (80 cm) from vertical. At its greatest, measured prior to 1990, the lean measured approximately 5.5 degrees. As of 2010, this has been reduced to approximately 4 degrees.