Against the fourth century apsidal wall, of which traces are found in the martyrium, were built in 858 two large columns to support the new choir above. At the same time the circular wall of the apse was pierced with windows. In order to support the apse of the upper church other two large isolated piers were built in 962, whilst the windows in the circular apse were blocked by a second strengthening wall. The same year saw the addition of a double transept at the commencement of the apse. Fulbert, in 1020, developed these transepts by carrying them out westwards almost to their present extent, and by so doing he left the altar dedicated to the Virgin, though unmoved, no longer before a thick wall, but stranded as it were in a corridor--a situation which aided, and still aids, the progress of the pilgrims past the shrine. The great bishop also extended the crypt by piercing the wall which closed the transepts towards the east, and making the ambulatory out of which opened the three large chapels with deep, round-headed windows (S. Joseph, S. John the Baptist, and S. Anne). To support the vaulting of this ambulatory a new wall was built round the martyrium. In the twelfth century, when the two western towers were constructed, Fulbert’s long galleries were extended and connected with them. The windows were raised and enlarged with the exception of those which were blinded by the porches of the upper church. Four smaller chapels, with pointed windows, were inserted between the large apsidal chapels of Fulbert, and these still exhibit traces of early thirteenth-century painting. At the same period, the transepts being now prolonged, it was found necessary to make two new flights of steps by which they might be entered. The beautiful doorway of the south staircase dates from this time. Lastly, when the porches of the Cathedral were completed, the transepts were connected therewith by means of vaulted passages, of which the one on the north side is near the Chapel of Notre-Dame-de-Sous-Terre, and the other southern one was turned into the Chapel of S. Nicholas in 1681. Entering the crypt by the south-eastern door, after noticing the windows of its apsidal chapels, you descend a stone staircase, and by the dim candlelight perceive on your left the long south gallery begun by Fulbert in 1020, and extended in the following century right up to the western tower. You are now in the original south transept of the crypt, and much of the masonry dates obviously from the tenth century. Turning to the left, the first chapel on the left is now dedicated to S. Martin. It was not originally a chapel, but in the twelfth century was used as an entrance to the crypt. Altered in the seventeenth century, as the windows and vaulting show, it was converted into a chapel in the nineteenth. It contains a few fragments of the original choir screen of the Cathedral destroyed by seventeenth-century vandals.