In the 10th century Paris was a provincial cathedral city of little political or economic significance, but under the kings of the Capetian dynasty who ruled France between 987 and 1328, it developed into an important commercial and religious center and the seat of the royal administration of the country. The Île de la Cité became the site of the royal palace and the new cathedral of Notre-Dame, begun in 1163. The Left Bank was occupied by important monasteries, including the Abbey of Saint-Germain-des-Prés and the Abbey of St Genevieve. In the late 1100s, the collection of colleges on the left bank became one of the leading universities in Europe. The Right Bank, where the ports, central markets, artisans and merchants were located, became the commercial center of the city, and the merchants assumed an important role in running the city. Paris became a center for the creation of illuminated manuscripts and the birthplace of Gothic architecture. Despite civil wars, the plague, and foreign occupation, Paris became the most populous city in the Western world during the Middle Ages.
Geography
The location of Paris was an important factor in its growth and strategic importance during the Middle Ages. Due to its position at the confluence of the Seine and the rivers Oise, Marne and Yerres, the city was abundantly supplied with food from the surrounding region, which was rich in grain fields and vineyards. The rivers also offered access for trading by boat with other cities in France and locations as far away as Spain and Germany. The Seine, without its stone embankments, was about twice as wide as it is today, and a tributary, the river Bièvre, entered the Seine about where the Jardin des Plantes is today. The largest island in the river, the Île de la Cité ("Island of the City"), was the easiest place to build bridges across the Seine; it became the crossing point on the important north-south trade route between Orléans and Flanders. The island was also the easiest place to defend; it gave the Parisians a sanctuary when the city was attacked by the Huns in the 5th century and Vikings in the 9th century. The Roman prefects had built their residences on the west end of the island; the first royal palace was built on the same site in the early Middle Ages. The first cathedral and the residence of the bishop were built on the east end of the island at about the same time.
The Romans had built their city on the Left Bank, because it was of higher elevation and less prone to flood; the forum was located on a hill about high, later called the Montagne Sainte-Geneviève after the patron saint of the city. In the early Middle Ages, the hill became the site of two important monasteries, the Abbey of Saint-Victor and the Abbey of St Genevieve, while another large and prosperous monastery, the Abbey of Saint-Germain-des-Prés, was built in the fields along the Seine farther west. In the Middle Ages, the monasteries attracted thousands of scholars and students who formed colleges that became the University of Paris in the beginning of the 13th century.
The Right Bank was swampy, but it was also the best place for landing boats. The gravel beach in which the Hôtel de Ville stands today became the port and the commercial center of the city, where the central market was located. The trade route from Orléans to Flanders passed between two large buttes on the Right Bank; the same route is followed today by the trains to Brussels and Amsterdam. The Romans probably built a temple to Mercury on the highest point at , which they called "Mount Mercury". It was the site of the martyrdom of Saint Denis and two other missionaries and thereafter was known as the "Mountain of Martyrs" or "Montmartre". During the Middle Ages, it lay outside the city walls, and was the site of a large convent and a pilgrimage church. During the course of the Middle Ages, the swampy land on the Right Bank was filled in and most of the city's growth took place there. This geographic distribution, with the administration and the courts on the island, the merchants on the Right Bank, and the University on the Left Bank, remained largely the same throughout the history of the city down to the present day.