The only thing in Roman history that ever filled the place of the police of today. The Urban Cohort, along with the Praetorian Cohort was the only infantry unit allowed in Rome itself, for security purposes of course. They were tasked with the upkeep of order in the ancient metropolis. That dosen't mean they were any less qualified to fight in the field, quite the contrary, they were highly adapt fighters and would provide a solid barrier to anyone invading Rome.