Not a lot is known about the development of wine during the dark ages. However, one thing is for sure; the water was definitely usually unfit to drink and both beer and wine were preferred alternatives. It is known that the monks in various Catholic monasteries in Europe kept the art of wine making alive. The monks made wine making a major daily activity. It was relaxing and was the source of the large quantities of wine they required for their religious ceremonies.
   As the Church extended their monasteries, they began to develop some of the finest vineyards in Europe and although these monasteries were the main source of wines for churches, it was often considered unchristian to drink it neat and get drunk, so the wine was often watered down when not being consumed as part of a ceremony. One assumes they had sources of clean water to do this with.
   Of course the wine was not watered down for communion because of the Catholics’ belief in transubstantiation, where the wine literally turns into the blood of Christ when consumed. This single belief is probably the sole thing that prevented wine from becoming a dying art during these times.
   One thing the monks were very good at was record keeping. They kept pages of data on all manner of things, and wine making was definitely one of them. They recorded their various recipes, improvements they had made, which grapes grew best in what conditions and how to ferment the best wines. In fact it was a monk called Dom Perignon who in the 1500s developed blending that was the basis of Champagne.
   As an aside, you may be interested to know that many monasteries, particularly those in Germany, also experimented with beer making during these times. And the German monasteries are said to have been the first to start adding hops to their beer. In fact we find the paths of beer and wine frequently cross each other through the ages.
   In southern Europe the Spanish wine trade became so developed under the Romans that by the second century, according to Roman records, some 20,000 amphoras of wine had been sent from Spain to Rome. But the decline of the Roman Empire and invasion of Hispania by northern barbarian tribes brought wine making in Spain to a temporary halt, although the subsequent arrival of the Visigoths prevented the art of wine making from dying out.
   The Visigoths (Muslim invaders who were officially forbidden by Allah to drink wine) surprisingly placed great significance on wine making and made many improvements, even though, ostensibly, they were cultivating the vine only for its unfermented fruit juice. It could be considered an irony that the old religion of Christianity kept wine making going in northern Europe, while the (supposedly) strictly tee-total new religion of Islam kept it going in southern Europe.