The bourgeoisie was a new social class created by capitalism. According to many working class citizens,

known as the proletariat, how did the bourgeoisie gain and maintain their wealth?

by exploiting the proletariat

Explanation:

for example, during the Industrial Revolution the "bourgeoisie" made large sums of money, but they paid the "proletariat" very little, and did not give them good working conditions, this argument is often used by Communists against the Capitalist, or Free Market, system.