You have served the community where you grew up, standing as its first line of defense against crime. You aren't a soldier, directing your gaze outward at possible enemies. Instead, your service to your hometown was to help police its populace, protecting the citizenry from lawbreakers and malefactors of every stripe. You might have been part of Even if you're not city-born or city-bred, this background can describe your early years as a member of law enforcement. Most settlements of any size have their own constables and police forces, and even smaller communities have sheriffs and bailiffs who stand ready to protect their community.
Use the tables for the Soldier background in the Player's Handbook as the basis for your traits and motivations, modifying the entries when appropriate to suit your identity as a member of the city watch. Your bond is likely associated with your fellow watch members or the watch organization itself and almost certainly concerns your community. Your ideal probably involves the fostering of peace and safety. An investigator is likely to have an ideal connected to achieving justice by successfully solving crimes.