The lovely romanesque nave and transept cannot prepare the visitor for the surprise awaiting them: the splendid Decorated gothic apse, the fabulous Chapel of the Virgin Mary and a 14C masterpiece, the octagon. The cathedral was begun in 1083; in 1250, the original romanesque apse was rebuilt with Purbeck marble. After the romanesque cross tower fell down in 1322, Alan of Walsingham designed the famous octagon, a triumph of medieval ingenuity.