The martyrium is the kernel of the crypt of Notre-Dame. The latter half of the tenth century was the period of vast crypts, and Vulphard, not content with merely renewing the martyrium of Gislebert, added a broad ambulatory round it, enclosed with a strong circular wall, which was broken by three advanced chapels. This was intended to make the martyrium, as the depository of the treasure and the Veil, safe from all accidents. We shall see presently how well it fulfilled its purpose. The church itself was extended westwards as far as the line marked now by the labyrinth in the pavement of the nave. Two great piers were inserted in the martyrium to support the raised choir.
In order to understand the growth of the Cathedral, which is in itself, to the eye that can see, an almost complete history of Christian architecture, it is necessary to form some idea of the church which Vulphard built. For in size and site it approached the modern building.
If we count the circular portion embracing the martyrium, and connected by stairs with the main building, it must have been over 100 yards long, and in breadth at least thirty, including the Chapel of Notre-Dame-de-Sous-Terre on the one side, and the sacristy on the south. But the nave and the aisles--the floor of which was on a level with that of Notre-Dame-de-Sous-Terre--were together only as broad as the modern nave by itself.
Like the tenth century Church of S. Martin-au-Val, the choir was raised ten or twelve steps above the nave, and was surrounded by an ambulatory. There was a double transept: and it may be that we can trace it in the recesses which now form the Chapels of S. Savinian, S. Potentian and the Saints Forts on the one side, and the Chapel of S. Clement on the other.
As to the details and ornamentation of the church nothing remains, but we can say from our knowledge of the times, and from a comparison with what tenth century work there is in Chartres, what could and what could not have been, if not what was. There may, for instance, have been a belfry, and, seeing that the great square tower of the Abbey Church of S. Père was at that time being built, there probably was. Perhaps two such towers flanked the two extremities of the façade. They would have had no spires, but a four-sided, pyramidal roof. Squat columns or square pillars, one may suppose, carried the series of plain round arches of brick and stone which connected the aisles with the central nave. But the great ‘Triumphal Arch,’ as it was called, separating choir from nave, was richly adorned with sculpture and painting.