Impractical. Those buildings would disintegrate every time the turtle moved and the first aid tent would be full of injured and queasy patients. No one would write. Have you every tried writing or drawing in a moving car? Pic is neat but the premise is shaky.
Oh,errrr...hmmm, maybe these folks built their cities on what they thought were lifeless giants. For centuries they lived peacefully on their backs, till one day, these giants began waking up one by one. Unable to live on giant moving creatures, most of these cities were abandoned or were reduced to ruins. So what we have left are the remnants of once great civilizations forever sitting in silence on the backs of giants.