The Mezquita's most striking feature is a design solution to a practical problem, and its most famous alteration is a monument its own patron reportedly regretted.
Arches stacked to gain height
Builders needed a tall prayer hall but had only short reused Roman and Visigothic columns to work with, and solved the problem by stacking two tiers of arches, alternating red brick and white stone voussoirs, on top of the columns, a design producing the mosque's famous striped double arcade and letting a low column support a much taller roof than its height alone would allow. The mihrab, added under Caliph al-Hakam II in the tenth century, is decorated with Byzantine glass mosaic, made by craftsmen reportedly sent specifically for the purpose by the Byzantine emperor at the caliph's request.
A cathedral built into the middle of it
In the sixteenth century, a Renaissance cathedral nave was constructed directly through the centre of the mosque's columned hall, a project undertaken with royal permission from Charles V, who is widely reported to have later regretted authorising it, on the grounds that the insertion destroyed something unique to build something that could have been built anywhere, a reaction frequently repeated in accounts of the building though not verifiable to an exact original wording.
One building, two structures still visible
The result is a single building in which the original mosque's forest of arches surrounds the inserted cathedral rather than either structure being removed to accommodate the other, a physical compromise that has outlasted the dispute over its wisdom.