What led to the growth of europe’s population in the middle ages?

Well in the middle ages after the decline of the Roman Empire, people were confused since the government and civilization as they knew it, was gone. Barbarians rule the land now, languages and religion is getting mixed with the savages and Roman citizens. Throughout all this chaos, there is a world wide panic from other countries that are depending on Roman grains and shipments. Allied factions and tribes now starve without the support of the Empire, free roman citizens now slaughtered and violated in the streets, and the worst part of it all is that the barbarians don't know how to live in the roman complex cities they built. All the Aqueducts and sewage systems now forgotten in time, the world everyone knew it, is gone. 

The world fell back in time, after the decline in population the world grew back slowly and around 800 AD it started getting better and the growth came back slowly. Since the barbarians that conquered the lands now turn to knights and kings, up rise kingdoms and castles and the population growth slowly comes back. 

Monarchy takes over the middle ages...

The answer would be advertising.