What explains the development of cities and towns during the Middle Ages

During the Middle Ages, cities where not very common. Cities where very hard to build, there needed to be a certain small town to be rich enough to expand. But, the big cities where developed by a leader, King or Queen. This leader had a large amount of money and would continually buy land and expand there territories. For a town to be successful they would need to be in a stable relationship with the big cities. The town folk would work the land in exchange for protection. But the town folk would also get unfairly taxed.