central aisle in front of the altar. Nearly completed, the cathedral was consecrated in 1061. This phase of construction, called Speyer I, consists of a Westwerk, a nave with two aisles and an adjoining transept. The choir was flanked by two towers. The original apse was round inside but rectangular on the outside. The nave was covered with a flat wooden ceiling but the aisles were vaulted, making the cathedral the second largest vaulted building north of the Alps (after Aachen Cathedral). It is considered to be the most stunning outcome of early Salian architecture and the "culmination of a design which was extremely influential in the subsequent development of Romanesque architecture during the 11th and 12th centuries". Around 1090, Conrad's grandson, Emperor Henry IV, conducted an ambitious reconstruction in order to enlarge the cathedral. He had the eastern sections demolished and the foundations enforced to a depth of up to eight metres. Only the lower floors and the crypt of Speyer I remained intact. The nave was elevated by five metres and the flat wooden ceiling replaced with a groin vault of square bays, one of the outstanding achievements of Romanesque architecture. Each vault extends over two bays of the elevation. Every second pier was enlarged by adding a broad pilaster or dosseret, which formed a system of interior buttressing. Engaged shafts had appeared around 1030 in buildings along the Loire (Saint-Benoît-sur-Loire, Auxerre, Loches) from where the technique spread to Normandy and the Rhineland. The only other contemporary example of such a bay system is in the Church of Sant Vicenç in Cardona, Spain. The "double-bay system" of Speyer functioning as a support for the stone vaults was copied in many monuments along the Rhine. The addition of groin vaults made the incorporation of clerestory windows possible without weakening the structure. "The result is an interior of monumental power, albeit stark and prismatic when compared with contemporary French buildings, but one which conveys an impression of Roman gravitas, an impression singularly appropriate for a ruler with the political pretensions of Henry IV." In the course of these modifications the cathedral was equipped with an external dwarf gallery, an arcaded gallery recessed into the thickness of the walls, and which is a natural development of the blind arcade. Such blind arcades were used extensively as decorations, lining internal and external walls of many Romanesque churches. At the east end of