[Andrew_nenakhov]

There is no equivalent only because there were no economy that could afford such projects. Kingdoms were much smaller and couldn't waste resources on grandiose vanity projects. People certainly knew how to make an aqueduct, or a road.

As for arenas, they came out of fashion with the rise of the Christianity. You know, publicly feeding early Christians to lions didn't help popularize this type of entertainment.

Also, all this knowledge was very well alive in Constantinople, so any rich ruler who wanted to build a replica of Hagia Sofia could just hire someone from there. The only problem, there weren't any such rulers who had enough resources.

Regarding Barbegal waterwheels, what do you suggest checking? Waterwheels were abundant in all medieval Europe, this knowledge was never lost.