Read this and answer the question. If the question is unanswerable, say "unanswerable".

The peaceful introduction of Islam in the early medieval era of Somalia's history brought Islamic architectural influences from Arabia and Persia. This had the effect of stimulating a shift in construction from drystone and other related materials to coral stone, sundried bricks, and the widespread use of limestone in Somali architecture. Many of the new architectural designs, such as mosques, were built on the ruins of older structures. This practice would continue over and over again throughout the following centuries.

In the medieval period, what buildings were notably built atop older ruins?
mosques