Presumed course of the Rubicon
(Wikipedia and its inaccurate accuracies)

The Rubicon was basically the border between Roman Italia and Gallic Italia (even afte the conquest of N.Italia).

The "Field of Mars" was were legions assembled and camped when near Rome. Though armies were not officially allowed to be deployed in Italia.

Rome had a 'police force' in prefects. They were men who fought fires. Though they normally fought the fire by knocking down the building. And they carried a kind of pick-axe with them to knock over buildings. They would also keep the peace (with their pick-axes). But for the most part, if you lived in Roma and had money, you hired your own guards. If you didn't have money you had to protect yourself. (Remember that throughout history, everyone carried a knife around with them as a multi-tool and protection.) In the Imperial Era, there were the Urban Cohorts. Sent in during large roits. I get the picture these guys didn't break up riots, but rather killed everyone roiting.