It's hard to know exactly how a medieval city of that population would look. We do know they weren't common. Only a few cities ever came close to that size. But we do know a few factors, and can extrapolate known data to get more information.
- Space: A city needs to have enough space to support its population. Only so many people can be crammed into a certain space. The higher the population density, generally the faster diseases spread. Calamities, such as fire, are more likely to occur in places with a higher population density. Modern towns in the U.S. with roughly 100,000 people have a population density of about 760 per $km^2$. So for a population of 75,000, you would need about 100 $km^2$. But in reality Medieval towns were generally less dense. If you use a population density half of modern cities, a city that large would require 200 $km^2$
- Materials: You need to be able to produce enough food for all those people. According to this site, one square mile (2.56 $km^2$) can support about 130 people. This means that for one city of about 75,000 people, approximately 3,808,693 $km^2$ of farmland would be needed. This could also be supplemented by the sea, if need be. For this to work, the city needs to be able to import food from many kilometers around. To facilitate this transport of food, several arteries of travel, such as rivers, need to come near or intersect the city. The city would also need to be able to get wood, stone, and other building materials. Wool or cotton would need to be available for clothing.
- Facilities: Cities have two basic needs as far as facilities go. First, is to get food and materials in, second is to get waste (bodily or otherwise) out. For this the city needs to construct sewages, transport system, and store houses.
- Trade: Cities need trade to exist. A large city needs to be a center of trade, so people are attracted to it. Such centers of trade are often on rivers or by the ocean. Water provides easy access via ships. Large cities also form near sources of commodities, because they promote trade.
- Protection: Cities are prime targets for raiding. People group together so they can better protect themselves. That is part of why the serf system existed in medieval Europe. Nobles protected their serfs against other nobles and robbers.