A large part of the job (at least in European and Byzantine cities) was to be on the watch for Fire. A small fire could be a real threat to entire cities until the 1900's. The night or fire watch would rally the population to fight the fire. Guards would also be sometimes stationed at gates for toll (tax) and image reasons. I am not a real expert on Medieval history although I have studied the history of law enforcement and firefighting. Watchmen in England since the 13th century would look for major breaches of the peace and summon effectively a 'posse'. They would usually have a section of a city or an entire town to somewhat patrol.