The Louvre began as a royal fortress in the late 12th century, while the main royal palace remained on the Île de la Cité. Charles V extended and improved the Louvre in the 14th century and installed in it his famous library of 973 books, the largest in France. It was not until the 16th century, beginning with Francois I and continuing under the reigns of Henri II and Henri IV that the Louvre became the main Palace. It remained so until 1682 when Louis XIV moved the main royal residence to Versailles. At the beginning of the 19th century, after the Revolution, the Louvre was transformed into a museum and gallery to house the royal collections accumulated over the centuries, as well as war booty captured by Napoleon in his campaigns, and more modern acquisitions. (The Mona Lisa, for example was in the collection of Francois I and so has been in France for about 500 years.) Since then, the Louvre has become one of the most famous and most visited art museums in the world. One visit on one day cannot do it justice.