[MENTION=17106]Ahnehnois[/MENTION]: I usually resolve this issue by assuming that type of mutually assured destruction exists. Smaug might be able to wipe the city of Bywater off the map, but doing so will encourage reprisals from the King, who will assuredly hire powerful adventurers to act on his behalf. Knowing this, Smaug limits his activity to the frontier, where his actions are less likely to bring violent retribution down upon his head.
On topic, I'd set a safe zone of about a day's march around a city this size. There are any number of small thorps and farming centers in the safe zone. The city either maintains a small garrison in some of these places, or its patrols stop at them overnight. Raiders from outside the safe zone are discouraged as much by the threat of retribution as anything else.
The city also makes an effort to root out monsters, bandits or other threats that attempt to set up lairs within the safe zone.
Of course, that doesn't mean the entire area is actually safe. Places unfit for agriculture are rarely visited and may offer refuge to dangerous creatures. Swamps, forests, moors, rocky hills, are dangerous even within the safe zone.
On topic, I'd set a safe zone of about a day's march around a city this size. There are any number of small thorps and farming centers in the safe zone. The city either maintains a small garrison in some of these places, or its patrols stop at them overnight. Raiders from outside the safe zone are discouraged as much by the threat of retribution as anything else.
The city also makes an effort to root out monsters, bandits or other threats that attempt to set up lairs within the safe zone.
Of course, that doesn't mean the entire area is actually safe. Places unfit for agriculture are rarely visited and may offer refuge to dangerous creatures. Swamps, forests, moors, rocky hills, are dangerous even within the safe zone.
