"A great city looms on the horizon."
Medieval cities were extremely small by our standards. Perhaps only 250,000 residents during the medieval period. Cities were geographically small with the average about 1 square mile with 250,000 inhabitants. The streets were exceedingly narrow and unpaved; mud was common. Sometimes the main street and market square were cobblestoned. Cities and larger towns were usually surrounded by a wall, which enhanced the separation between urban and rural, but the fields frequently came up to the wall.
The walls are what really defines a medieval city, especially a fantasy one. There is always the threat of invaders, monsters, and war. Within the walls, people can feel safe enough to invest in buildings, colleges, churches, and business enterprises. Entering a city isn't always a guarantee. Those entering the city immediately gain safety and access to many customers and many services, but what does the city gain? So tolls are common (or bribes). Some cities may prevent weapons from entering so those in power can stay in power.
Rarely does a city have only one wall. Within a city there is often a wall to divide the rich from the poor areas. And within the rich area, a palace or cathedral would have its own defensive walls.
Each city has its own culture, landmarks, and unique history. A good dungeon master can turn the brief description above into a fantastical setting full of intrigue and adventure.
Contents of chamber pots were emptied into the streets. With mud streets this presented a messy problem. With a heavy rain one could hope for a flushing action to wash the excrement to the river. A light rain only added to the problem. This was a health problem; polluted springs and wells were common. The most commonly consumed beverages were not water but wine and beer. Beggars, who were seen as social victims, abounded. Disease was viewed as punishment. Smallpox was endemic, leprosy was common and the victim was segregated. Luckily, in D&D there are always clerics to heal the sick. Though if a priest can only remove one or two afflictions per day, he is going to heal the ones who pay the most money or whom are most valuable to the church.