included many small chapels throughout the building and various relatively cosmetic changes. The most substantial and visible additions are the cruciform nave and transept of the Capilla Mayor (the main chapel where Mass is held today) which were begun in the 16th century and inserted into the middle of the former mosque's prayer hall, as well as the remodelling of the former minaret into a Renaissance-style bell tower. The hypostyle hall The mosque-cathedral's hypostyle hall dates from the original mosque construction and originally served as its main prayer space for Muslims. The main hall of the mosque was used for a variety of purposes. It served as a central prayer hall for personal devotion, for the five daily Muslim prayers and the special Friday prayers accompanied by a sermon. It also would have served as a hall for teaching and for Sharia law cases during the rule of Abd al-Rahman I and his successors. The hall was large and flat, with timber ceilings held up by rows of double-tiered arches (arcades) resting on columns. These rows of arches divided the original building into 11 aisles or "naves" running from north to south, later increased to 19 by Al-Mansur's expansion, while in turn forming perpendicular aisles running east–west between the columns. The approximately 850 columns were made of jasper, onyx, marble, granite and porphyry. In the original mosque, all of the columns and capitals were reused from earlier Roman and Visigothic buildings, but subsequent expansions (starting with Abd al-Rahman II) saw the incorporation of new Moorish-made capitals that evolved from earlier Roman models. The nave that leads to the mihrab – which was originally the central nave of the mosque until Al-Mansur's lateral expansion of the building altered its symmetry – is slightly wider than the other naves, demonstrating a subtle hierarchy in the mosque's floor plan. The double-tiered arches were an innovation that permitted higher ceilings than would otherwise be possible with relatively low columns. They consist of a lower tier of horseshoe arches and an upper tier of semi-circular arches. The voussoirs of the arches alternate between red brick and white stone. Colour alternations like this were common in Umayyad architecture in the Levant and in pre-Islamic architecture on the Iberian Peninsula. According to Anwar G. Chejne, the arches were inspired by those in the Dome of the Rock. Horseshoe arches were known in the Iberian Peninsula in the