How and why was spolia incorporated into the design of the Great Mosque of Kairouan?

What will be an ideal response?

The ideal Answer should include:
1. At the Great Mosque of Kairouan the marble and porphyry columns from antiquity are used in the prayer hall and the courtyard.
2. The columns and capitals do not always match because they come from various Roman temples and Byzantine churches that were dismantled during the seventh century.
3. The round arches of the arcades appear to be Roman, but the builders modified them to be horseshoe arches.
4. By constructing the mosque with elements of Roman and Byzantine buildings, the mosque documents the Islamic victory over Roman and Byzantine empires, but, by preserving these ancient columns, the mosque also shows respect for the architectural traditions of earlier inhabitants and rulers.