That term “Medieval” gets used too loosely for my taste. Greek and Roman knowledge was lost during the ‘Dark Ages’ or Early Middle Ages, which ended in 700AD. The Great Library at Alexandria was destroyed no later than this date.
The distinction is important because in the true Medieval Period (1000 to 1300) Europeans built the great Cathedrals, using techniques the Romans never knew.
Generally corresponding with the Medieval Warm Period.
Global Warming is your friend.
...and what was happening during the ‘Dark Ages’?
The term is a pejorative and quite subjective. The collapse of Rome was a good thing for many peoples.
Many of the mechanical devices, or drawings thereof, designed during the Greek and Roman empires were not known in the Middle Ages.
Had the people of the Early Middle Ages had Roman water supply (aqueducts) and sanitation design, much disease may have been prevented.
As far as civil engineering, concrete formulas were either not known or weakened with inferior materials during the Middle Ages. The structural properties of the Pantheon from the Roman period made it one of my favorites even though I have personally visited the Gothic cathedrals in Amiens, Chartres, Le Mans, Rheims, and Paris. It’s an impressive dome when you consider how many years it came before Brunelleschi’s design for the Duomo in Florence.
Gothic cathedrals were an improvement upon Romanesque designs, their main achievement was the flying buttress, at first incorporated in a trial and error basis. The flying buttress (Le Mans has really big ones) allowed greater height and window size.
The "Dark Ages" were just in Western Europe. The Eastern Roman Empire, based in Constantinople, kept going, and preserved much of Greek and Roman knowledge. It was just unavailable to Western Europe for much of this time due to Islamic piracy making sea commerce difficult.
Whence came this knowledge? One thing is certain: it came from somewhere.