This unassuming little building here is known as Mezquite del Cristo de la Luz, and is over a thousand years old. Today, it's the only mosque still standing in a city that once had ten. It was built in the late tenth century, and I know this because as soon as you step in through the arches in the front, you can see a Kufic inscription on the pillar, which dates it to ""Muharram in the year three hundred and ninety"" of the Islamic calendar, which corresponds to the late tenth or the early eleventh century. The inscription also tells us that it was commissioned by a man named Ahmad ibn Hadidi, and designed by the architect Musa ibn Ali, assisted by someone named Sa'ada. A thousand years later, we still know their names. That's not bad for a building the size of a small chapel. Now, look at the exterior. The brick arches and the geometric patterns are pure Caliphal architecture, a miniature echo of the Great Mosque of Córdoba. But the real magic is what's inside. However, you need to buy a ticket and wear modest clothes to step inside. The interior is only about eight metres square, but it contains nine bays, each covered by a vault with a unique geometric rib design. It's like a sampler of Islamic architectural genius compressed into a single room. And here's where the layers start. Three of the four interior columns have Visigothic capitals, meaning carved stone tops recycled from buildings that stood here centuries before the mosque was built. So you've got sixth-century Visigothic stonework holding up a tenth-century Islamic prayer hall. Then, there is a rounded extension at the back that was added after the Christian conquest in the late twelfth century, making it possibly the earliest example of Mudéjar architecture anywhere. If you don’t know what Mudéjar architecture is, it is the distinctive style where Christian builders adopted Islamic techniques. Inside the mosque, you can still make out faded murals of a Christ Pantocrator. Now, remember Alfonso the Sixth from the gate? This is where his story gets its most famous scene. According to legend, as the king rode up this road on the day of his conquest, his horse suddenly knelt in front of the mosque and refused to move. Alfonso took this as a sign. He ordered the wall opened, and behind it they found a small crucifix with a lamp still burning, a flame that had supposedly been kept alive for the entire Muslim rule. The crucifix was called the Cristo de la Luz, the Christ of the Light, and it gave the mosque its Christian name. Is the story true? Historians call this a 'foundation myth', a story used to explain why a mosque became a church. But Alfonso reportedly left his shield here with an inscription claiming that the first Christian Mass of the Reconquista was celebrated in this spot. The lamp may or may not have stayed lit for three hundred years, but the legend has stayed alive for nearly a thousand years. You're now literally retracing Alfonso's route into the city. Follow the map, and in a few minutes you'll reach a stunning gate with horseshoe arches. That's our next stop.