These followers of Catiline contrasted with the defenders of the state. The issue of such a contest cannot be doubtful.
urbes coloniarum, etc.: the colonies and free communities (municipia) included the walled cities (urbes) in their territory. These well-manned walls would be more than a match for Catiline's rude works (tumulis).
causas, i.e. the cause of the conspirators and that of the state in their moral aspect (cf. in eius modi, etc., 1.12, below). ex eo ipso, from the very comparison. bona ratio, good counsel; perdita, desperate.