The Loire Valley, which shares the northern boundary of French wine production with Champagne, is famous for its mild climate and beautiful Renaissance castles. The river, once a major trade route, but also a means of access for invaders, has played an important role in the history of the region. The development of its vineyards parallels that of Christianity, from St Martin (4th century), Bishop of Marmoutier to the ruin brought about by the wars of religion. It was also the heart of artistic development in the 16th century with Ronsard, Du Bellay, Rabelais, Descartes, and Leonardo da Vinci. Four viticultural regions can be counted: the Nantais region, the Anjou and Saumur region, Touraine, and the Centre, producing mostly white wines and light fruity red or "clairet” wines which result from the short maceration of red grapes.