As I'm sure you know, if only from the MGM movie, the Emerald City is the capital of the Land of Oz. It's also the only real city in the country. Sure, there are other places with "city" in the names, but they'd all be small towns by our standards. The Munchkin capital, the Sapphire City, has only around 1000 inhabitants, and I suspect most of the rest have less. The city of the Skeezers has a population of a mere hundred. The Emerald City, as per The Emerald City of Oz and The Gnome King of Oz boasts a population of 57,318, and I doubt it's changed much since then. While officially built by the Wizard of Oz, he doesn't appear to have worked totally from nothing. According to Pajuka in Lost King, Pastoria had a palace where the Emerald City now stands. My guess is that whatever settlement was there was largely destroyed by the Wicked Witches, and the Wizard had his subjects build on the ruins. It's definitely a planned city, rather than one that grew up gradually, and there's a high wall around it with four gates. One odd detail revealed in Wonderful Wizard is that it only appears green when inhabitants and visitors wear green glasses, but when the glasses are abandoned in later books, there's no indication that its color was diminished. Emerald City tells us, "The Emerald City is built all of beautiful marbles in which are set a profusion of emeralds, every one exquisitely cut and of very great size. There are other jewels used in the decorations inside the houses and palaces, such as rubies, diamonds, sapphires, amethysts and turquoises. But in the streets and upon the outside of the buildings only emeralds appear, from which circumstance the place is named the Emerald City of Oz." Was the city redecorated during Ozma's reign, or what? We don't know. Another oddity regarding the city is that it wasn't until John R. Neill took over as Royal Historian that any of the streets were named. Neill chose primarily alliterative names related to food: Strawberry Street (a street name I've occasionally come across in our own mundane world), Banana Boulevard, Doughnut Drive, Celery Street, Apple Alley, and Pumpkin Place. An old Baum Bugle article included a diagram of the city based largely on Neill's text and drawings, but while I do have the article somewhere, it's not readily available at the moment.
The most popular image of the Emerald City is the one in the movie, but I've never been particularly satisfied with how it came out. Actually, I like the pictures of the capital in the distance, which maintain the domed houses that L. Frank Baum says are the norm in Oz. What I'm not so keen on are the scenes that take place inside the city, which indeed make it look green and richly decorated, but it comes off as more like one building than a whole city. The one building that the cast visits other than the palace is the Wash and Brush-Up Company, which looks like a room. And when Dorothy and her friends are waiting outside the palace, they use a carpet to simulate a royal robe for the Cowardly Lion during his "King of the Forest" number. Unless there's an awards ceremony going on, how often do you see carpets outdoors?