What does the proliferation of trading routes and cities suggest about the state of the medieval European economy?

The proliferation of trading routes and cities suggests to us that the European economy at this time was relatively strong.  Both trading routes and cities depend on a strong economy for their existence.

If the economy is not strong, cities cannot exist.  In order for cities to exist, two things have to be true.  First, farms in the area of the city have to be prosperous.  If farms are producing a lot of food, they can sell to people in the cities.  If the farms are not producing well, cities cannot arise because there will not be enough food to support people who live in cities.  Second, there have to be jobs available in the cities.  People cannot live in cities unless there is something for them to do to make a living.  In the country, people can farm.  In the cities, they cannot do this.  They have to have some other sort of employment in order to survive.  What this means is that the proliferation of cities means that farms were doing well and that there were jobs to be had in the cities.

If the economy is not strong, trading routes cannot exist.  Trading routes, of course, need goods that can be carried along them.  If the economy is weak, it can only produce enough goods to satisfy its own consumers.  In addition, it does not have any excess wealth to use to buy trade goods from others.  If the economy is strong, people in any given place can produce enough surplus goods to trade with people elsewhere. 

Thus, the proliferation of cities and trading routes tells us that the European economy must have been relatively strong during the portion of the Middle Ages depicted in this map.