Before any detailed consideration of the architecture of the cathedral, it is well to be clear as to the various dates of the chief parts. But it must here be remembered that practically in every instance the now existing portions replaced still earlier structures on the same site. Mention has been made already of the changes from the original building to the one commenced in the eleventh century. In 1079 Bishop Walkelin laid the foundations of a great Norman church, of which the transepts, the outer face of the south nave wall, the core of the nave itself, the crypts, and a portion of the base of the west front are still existing. Walkelin's work was completed in fourteen years, just before the end of 1093. The tower fell in 1107, but was rebuilt soon afterwards in the form which we now see it. Bishop de Lucy's work, which came next in date (1189-1204), includes the Chapel of the Guardian Angels, flanking the Lady Chapel, at the north-east end of the cathedral, and the corresponding chapel on the south-east, which afterwards became the chantry of Bishop Langton. The piers of the presbytery probably date from about 1320. The west front was rebuilt in Edingdon's time (1345-1366), and a small part of the reconstruction of the nave, the first two bays of the north aisle, and a bay of the south are generally attributed to him. The great re-modelling of the nave, the outer walls of the presbytery, and the continuation of the Lady Chapel range in date of completion from the end of the fourteenth to the sixteenth century. So much, however, of each period has been altered, and often modified almost beyond recognition by later additions, that it is impossible to make more than a rough guess at the age of the various portions. The work of Wykeham and his successors is so important that it must be left until we reach it in its proper place.