Mezquita-Catedral de Córdoba audio guide,
five stops, free.

Stand with your back to the street and look at the wall, because this is one of the oldest faces of the building. The tall studded door sits inside a horseshoe arch, and the arch sits inside a rectangular frame. Red and cream stones run round the curve by turns. That little arrangement, door, horseshoe, frame, became the model for the Moorish gateways that followed, and this door is the earliest one still standing that has all three.

Its first name was Bab al-Wuzara, the Viziers' Gate. Across this street stood the emir's palace, and this was most likely the door the ruler and his officials used to walk from their offices into the mosque. Later rulers made that walk private: a covered bridge, a sabat, was built over the street further south so the ruler could step from palace to prayer hall unseen.

The man who began all this was a refugee. Abd al-Rahman the First was one of the last survivors of the Umayyad family, who had ruled the Islamic world from Damascus until they were overthrown and almost all killed in 750. He escaped to North Africa, took Córdoba, and in 785 started his mosque. It went up in about a year, fast, because the builders took columns and capitals from Roman and Visigothic ruins. Nobody knows the architect's name.

That first mosque was roughly square, 74 or 79 metres a side depending on whose measurements you trust. Half of it was open courtyard, half prayer hall, and this door opened into the middle of the prayer hall's west wall. The decoration you see was finished in 855 under the emir Muhammad the First, and his inscription in angular Kufic script, cut into the lintel and under the arch, is the oldest Islamic writing left in the whole building.

One small correction to the name. Everyone calls it the Door of Saint Stephen, San Esteban. The historian Manuel Nieto Cumplido argued that this is a mistake that simply stuck, and that the proper Christian name is San Sebastián, after a hospital of San Sebastián that once stood opposite.

Look up before you move on. Those heavy buttresses and the stepped battlements make the mosque look like a fortress from outside, and the surprise is kept for the inside.

Stop just inside and let your eyes adjust, because the hall is dim after the courtyard and the effect needs a moment. Columns run away from you in every direction, and above each one an arch, and above that arch another. People have called it a forest of columns and a hall of mirrors, and both are right: turn a few degrees and a new avenue opens.

Look at one column closely and follow it up. On top of its capital sits a horseshoe arch. But the column also carries a squared block of masonry, and from that block a second, round arch springs higher still. Two storeys of arches, with the ceiling on the upper one. The likeliest reason is practical. The builders were reusing Roman and Visigothic columns, and they were not tall enough to lift a roof to the height this hall wanted, so the builders stacked a second tier on top, the way Roman aqueducts in Spain already stacked their arches. There is a prettier theory, that the arches recall the palm groves of Syria that Abd al-Rahman the First had fled, and it is only ever offered as a suggestion.

The red and cream stripes are brick and stone. Keep an eye on them as you walk, because they tell you which century you are in. The first mosque had eleven aisles and about 120 columns, every one of them second-hand. Abd al-Rahman the Second pushed the hall eight bays south in the 830s and 840s, and his workshops began carving new capitals rather than only reusing old ones, and stopped giving the columns bases. Later, in 987, the vizier Almanzor added eight whole aisles on the east side, 47.76 metres of new hall, and made this the largest mosque in the Muslim world outside Abbasid Iraq. His part is plainer: over there every voussoir is limestone, and the red is painted on. His masons also scratched their marks into shafts and capitals, the builders' own signatures.

Add it all up and there are about 850 columns now, in jasper, onyx, marble, granite and porphyry, inside walls enclosing 180 by 130 metres.

The floor has changed too. At the start it was a thick layer of mortar over rammed earth, and everyone on it faced south, the way you are about to walk.

This is the qibla wall, the one every worshipper in the hall once faced, and the gilded doorway in its middle is the mihrab. Stand back far enough to take in the whole façade: a horseshoe arch glittering with gold, framed by a band of gold letters on deep blue, with carved marble panels on either side, and through the opening, a small pale room.

That room is the surprise. A mihrab is normally a niche, a shallow recess that shows the direction of prayer. This one is a little chamber of its own, octagonal, the Cabildo says, roofed with a dome shaped like a scallop shell. It is the earliest known mihrab that is an actual room. Look at the arch, and then think back to the Puerta de San Esteban on the street outside: the same horseshoe inside the same rectangular frame, a century on and made of gold.

It was built by the caliph al-Hakam the Second, who from 961 pushed the prayer hall another 45 metres south, twelve more bays of the same double arches. Around his new mihrab he fenced off a maqsura, the space kept for the caliph, with screens of interlacing, many-lobed arches. The mihrab and the domes in front of it were finished in late 965, and an inscription names four of the craftsmen, men who also worked on the reception hall of the caliph's palace city, Madinat al-Zahra.

The gold came from Constantinople. The chronicler Ibn Idhari tells how al-Hakam wrote to the Byzantine emperor, Nikephoros the Second, asking for someone who could lay mosaic. The emperor sent a master, and with him, as a present, about 1,600 kilograms of little glass and gold cubes. The master trained the caliph's own men, and between them they finished by late 970 or early 971. Scholars read it as the Umayyads of Córdoba reaching back to their ancestors in Damascus, whose great mosque, the same chronicler says, had been decorated with Byzantine help.

Read the letters in the frame, even if you cannot read Arabic. The largest band begins with verses of the Qur'an and turns into a thanksgiving, to God, for choosing al-Hakam for this venerable building. Inside the opening, the two pairs of slim columns are older than everything around them: they were taken from the previous mihrab, Abd al-Rahman the Second's, and set in here.

One odd fact about direction. From Córdoba, Mecca lies to the east-south-east, but this wall faces south. The mosque followed the street grid of Roman Córdoba, and a western tradition held that a qibla to the south was allowed, and none of the later extensions changed the line.

After 1236 this became the Chapel of San Pedro, and for centuries an altarpiece stood in front of the mihrab and hid it. In 1815 Bishop Pedro Antonio de Trevilla had the chapel taken apart. What came out from behind it was damaged, and Patricio Furriel repaired the mosaics in 1817 and 1818; the lowest strips of gold on this façade are his. Most of the gold above them has been catching the light since the tenth century.