The “Middle Ages” period is defined as the time after Roman hegemony centered in Rome itself. After Rome was overrun by Goths and ceased to control its empire, Constantinople was called the New Rome or Eastern Rome. Its cultural domination extended over the eastern Mediterranean, but not past Greece. Western Europe was no longer dominated by any one single people, but instead it was unsettled and wild. Various groups from the north contested for power.
Let’s look at the map before Rome’s influence ended. (link to Wikimedia map) On this map, Mediterranean culture has spread as far north as it could. There’s one large block of geography resistant to Roman culture, in the areas of modern Germany and Poland. Between 200 and 600 AD, most of these groups pushed east and south, taking over warmer lands. The Vandals and Lombards went to Italy. The Visigoths settled in Spain. Burgundians went to France. Some Saxons and Angles (not shown on Wiki map) moved into Roman Britain.
Medieval fashion began with two very divergent poles: Byzantium (Constantinople) with its Roman-like robes and wealth and the uncouth Germanic people who wore pants. Medieval fashion history is about the influence of the East on the West in repeated cycles, with the West each time altering the East’s clothing notions into forms more appropriate to their hard northern winters.
So in considering medieval clothing, we have to look at two radically different ways of covering the human body. Byzantium was relatively warm and already a crowded urban center. Northwestern Europe was forested with only scattered human settlements; wolves and bear roamed through the snow. Byzantium’s fashions were based in gold and imported silk; Germany’s fashions centered around fur. Byzantines wore slippers, robes and tunics; Franks and Saxons wore pants and boots, with many layers of wool and fur on top.
Flax was Europe’s native fabric material. Wool-producing sheep and goats, brought north from the Mediterranean, were a welcome addition. Silk was a rare import from China, not at first known outside the huge urban center. Cotton was yet unknown.