Suppose, for instance, you lived in a world filled with nasty monsters and other creatures of the night, all of whom wanted to kill you and eat you in various ways. Now, the good news here is that we all know such creatures cannot cross running water. So, if you're in danger, all you need to do is hop over a running stream and you're fine.
Now, if we take this to it's logical extreme, the best thing to do is to build a city on a river so that it'll always be surrounded by running water. Or, (because this is going to be in a fantasy novel and what's the point of fantasy novels if they aren't fantastic) build your city between twin gigantic waterfalls.
To clarify, this is how the waterfalls works. They are both fed by a single gigantic river, the river itself it incredibly wide and deep and has hundreds of tributaries making it up. At a point, the river splits into two equally sized smaller rivers, both of which split off and then curve around after about a few mile until they're parallel to the original river's course. At that point they continue on for a good ten miles until they reach the twin waterfalls. Thus you have a nice square patch of land to build a decent-sized city, at least by Dark Age standards, cheerfully protected from all the nasty things that go bump in the night thanks to your waterfalls.
There's only a single problem here, and that is sound. You see, waterfalls can get very loud by nature - Niagara Falls, for instance, has a sound level of around 90 dB, equivalent to that of a lion's roar. Now, no matter how loud the falls the people living in the city don't care - a little noise beats dying. But they're definitely going to want to make a few adjustments to day-to-day life to make living around the waterfall easier.
Assume a Dark Ages level of technology, and assume a noise level similar to that of Niagara Falls. What kind of adjustments would you expect the inhabitants of this city to make to cope with the constant noise of the waterfall?