Yeah, RobA makes some excellent points. Only well organised countries or empires will probably maintain a more elaborate road system.
During the time of the Roman empire there we some excellent roads in Europe for example, some of the highways made of stone. They were mainly created to allow the Roman legions to travel, but merchants could use them as well. The Romans could do this as there was a centralised government, with a large budget. Now, in the Middle Ages there were a lot of lords and kings and Europe was divided in hundreds of little states. Not many lords wanted to pay the costs of maintaining roads and certainly not all the lords in one region. The Roman road system fell into heavy disrepair and stone roads outside the cities became virtually non-existant. Travel was very slow, especially with cargo, and the roads could be unsafe as well. River travel was very important, especially for bulk cargo. But of course all the different lords and kings tended to want toll for use of the river system
Places where people can cross rivers are quite rare (bridges are expensive to build and maintain), so you can bet towns will spring into existance near those points.