The Emerald City is not only at the heart of Oz (being right in the middle of the four major countries of Oz), but it is also the heart of most of the Oz stories. Quite a few of the stories begin there and nearly all of them end there.
A few months ago, I spent a couple of posts looking at the Emerald City as a place for your characters to hang out and have adventures in. Rather than simply regurgitate that material, I think I'll just provide some links.
The Emerald Sandbox
This is the (Emerald) City
And if you are interested in purchasing Adventures in Oz: Fantasy Roleplaying Beyond the Yellow Brick Road, you can buy it at DriveThruRPG in print and PDF!