The cathedral was begun at about the same time as Chartres Cathedral, but the basic plan was very different. Whereas at Chartres and other High Gothic cathedrals the two collateral aisles were the same height as the nave, at Bourges the collateral aisles were of different heights, rising in steps from the outside aisle to the center. The old nave was preserved for a time to allow worship until the new choir was finished in about 1214. Then work began on the five vessels, or aisles of the new nave. The cathedral was complete enough by 1225 to be able to host a large council condemning the heresy of Catharism. Major work on the nave was finished by 1235, with the installation of the rood screen, which separated the choir from the nave.