What spurred the growth of cities in the middle ages?

The growth of cities in the Middle Ages was initially spurred by the establishment of merchants in places of safety, such as forts or near to Castles. This in turn attracted artisans, and then carpenters, bakers, cloth makers, etc., that came to these settlements to cater to the needs of this people. Eventually these settlements transformed into evergrowing cities.

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a and b!

dfroos means which is a and b