What were guilds, and why did they become controversial in the 18th century?

Guilds: they are medieval association of workers and craftsmen which ensure mutual aid and quality work in a particular production process in which they hold a monopoly acquired from the state.

During the mid-eighteenth century, they became the center of criticism by the followers of free trade who believed that restriction of work by the sate should be ended for a more competitive economic structure. By the mid-nineteenth century, the monopoly of guilds seemed to be disappeared.