Fantasy novel about a Castle perched at the edge of the World?

[Beatrix Kiddo]

While perusing the fantasy section of a bookstore several months ago I happened across a novel that contained a striking map of a Castle built on an island at what appeared to be the very edge of the world. An Ocean or perhaps a River flowed all around the Castle / City before terminating in an end-of-the-world waterfall.

Imagine a medieval keep built right on the lip of Niagra Falls. There might have been a long narrow road / drawbridge connecting the Castle with the mainland.

I received the impression that the novel took place within the confines of this fantasy city.

So, I was wondering if anyone knew the name of this particular fantasy Novel. And whether they would recommend reading it.

Thanks!