Parties assemble for the purpose. He may well have made many of these during his time in Turkey so that, in the absence of parallel accounts, we may use it as a model for what happened in Turkey - a model easily confirmed by observation. The civilization of the city then recedes, and its inhabitants decrease in number. This entails a decrease in the crafts. The materials that are in existing buildings are re-used for building and refinishing.
People then return to the Bedouin way of building.
They use adobe instead of stone and omit all ornamentation. The architecture of the city reverts to that of villages and hamlets. The mark of the desert shows in it. This is how God proceeds with his creatures. The perceptive comments of Ibn Khaldun are, I repeat, unmatched by contemporary accounts from Turkey.