A Simple Workaround for Monsters Entering your Cities:

Simply make the city a different map than your wild lands. For example, you can add a gate which leads into your city or town. Of course, this may remove some realism, but it is a good workaround for new developers. It doesn't solve the issue of monsters waiting for you outside, but it's called a workaround, not a fix.

Adding multiple gates to the city could be a work around for monsters waiting for you. Simply enter through one side and exit through another

True, though that seems like more of an obvious thing.
And cities may have to be big or irregularly shaped so they didn't go to the next gate when you appeared. Either way, it is a simple little fix.