They were forced in certain professions, such as banking, by making anything else unavailable to them. A very unlikeable profession for your average Joe, as having to interact with a banker is often bad news for people. They were also forced to keep to themselves in specific parts of the city -- combine that with looking different (mostly in hair and clothing), religious customs other than what the majority population was used to, difficult dialects bordering on being a different language altogether. The perfect scapegoat storm, so to speak, whenever there are fits of xenophobia.