A cathedral is a big church that is usually under the control of a bishop. It is usually the seat of a diocese, a church district headed by a bishop. Most cathedrals, especially medieval ones, are located in the centre of a town or city. They served as the centre of public life. Markets, meetings and masses were held there. Most cathedrals are built in the shape of a cross. The main entrance is at the west end at the bottom of the cross. There is a long central aisle called the nave and two side aisles. The arms of the cross are the transepts and meet the nave at the crossing. Towers and domes were often built over the crossing. The altar and the seats for the choir are at the eastern end of the nave in front of the apse, which looks like a semi-circle. Cathedrals have been built in almost every architectural style. But most of the famous European cathedrals were Byzantine, Romanesque, Gothic or Renaissance. The greatest era of building happened in Europe between 1000 and 1500 A.D. Cathedrals were filled with carved sculptures .The walls had paintings and the windows were painted with scenes from the Bible and the lives of the saints. In the first centuries A.D. the centre of Christianity was around the city of Byzantinium, which was later called Constantinople and is now Istanbul. Architecture in the Byzantine Empire was a combination of styles that came from the Middle East and ancient Rome. Hagia Sophia was one of the first Byzantine cathedrals. As time went on the centre of Christianity moved to Europe. Rome had become completely Christian other countries to the north also converted to Christianity. The new style came up at around 1000 A.D. Northern art began to influence traditional Roman art. By the 1100s people the Germanic peoples from the North-Germans, Goths and Franks-dominated European Christianity. Their cathedrals were very tall as if they wanted to reach to heaven. Most of Europe's famous Cathedrals are built in Gothic style but every region had its own style. In the 1300s a new spirit began to grow in Italy. People were curious about new things and made new discoveries. The period was called Renaissance, which means rebirth. It was a time when the population wanted to know and learn things about classical Greece and Rome. Renaissance architects looked back to the buildings and structures of ancient Rome and Greece. They mixed features of old and new architecture. St.Peter's cathedral in Rome is one of the best examples of a Renaissance cathedral. The dome was designed by Michelangelo. Many cathedrals of Western Europe are famous for their great beauty. France is the home of some of the most beautiful cathedrals. St.Paul's Cathedral in London and Canterbury Cathedral are England's biggest churches. Other famous cathedrals in Europe are in Cologne, Milan and Seville.