divided between a hypostyle prayer hall to the south and an open courtyard (sahn) to the north. As the mosque was built on a sloping site, a large amount of fill would have been necessary to create a level ground on which to build. The original mosque's most famous architectural innovation, which was preserved and repeated in all subsequent Muslim-era expansions, was its rows of double-tiered arches. Much speculation has been offered on the inspiration for this particular design, including the alleged similarity of the arches to a forest of palm trees from Abd ar-Rahman's youth in Syria; however, a more technical motivation may have been the fact that the available columns being reused from previous buildings were not tall enough on their own to raise the ceiling to the desired height. The precedent of multi-tiered arches was also already present in the Iberian Peninsula thanks to remaining Roman aqueducts (e.g. the Milagros Aqueduct at Mérida). The mosque's original mihrab (niche in the far wall symbolizing the direction of prayer) no longer exists today but its probable remains were found during archeological excavations between 1932 and 1936. The remains showed that the mihrab's upper part was covered with a shell-shaped hood similar to the later mihrab. The mosque originally had four entrances: one was in the center of the north wall of the courtyard (aligned with the mihrab to the south), two more were in the west and east walls of the courtyard, and a fourth one was in the middle of the west wall of the prayer hall. The latter was known as Bab al-Wuzara''' (the "Viziers' Gate", today known as Puerta de San Esteban) and was most likely the entrance used by the emir and state officials who worked in the palace directly across the street from here. The outer walls were reinforced with large buttresses, which are still visible on the exterior today. The courtyard of the mosque was planted with trees from at least as early as 808, according to the jurist Ibn Sahl, who himself cited five previous authorities. Although the species of tree is not known, the fact that these were fruit trees is attested in Ibn Sahl, who was consulted as to whether such a garden was forbidden and, if not forbidden, whether it was permitted to eat from it. That the trees remained in the courtyard is demonstrated by two seals of