So I learned quite a long time ago that Sparta had a policy that Rome later adopted where every soldier had to lay down their arms and armor before entering the city, therefore making them citizens and not a soldiers.
Am I missing some very simple answer to this because I'm thinking that there are no garrisons to keep the peace in the cities.
The Romans did have a garrison for the city of Rome. The law of soldiers not being allowed into the city with arms was for soldiers returning from campaign. The Urban Cohorts, who acted as a police force in Rome would have served as a garrison in times of need. As well as this, citizens would take up arms against invaders as a Militia if the city did not have another line of defense.