Here are three maps:
1) Rome. The yellow zone was enclosed by the Servian Wall, the one Hannibal would have faced. When they were built, Rome wasn't CAPVT MVNDI yet, but a fairly powerful city-state.
2) Athens. The red lines are fortifications, stretching from the city to two harbors. They were called the Long Walls for a good reason, and protected sea trade routes and agricultural lands, essential during a siege. It's not the kind of wall anyone could have afforded, though, and Athens' thalassocratic empire was at the peak of its power.
3) Syracusae. The red line is the Dionysian Wall, a powerful defensive work that extended the fortified zone to the most peripheral districts of the rich Greek colony.
Scale is given, so judge the dimensions for yourselves, keeping in mind these examples are larger than average. There were fortified towns and strongholds much smaller than that.