As you can see from the pictures, it's the Emerald City!
"The Emerald City, is the largest and most relevant city in the magical Land of Oz. It stands in the very center of Oz, being the official imperial capital of the entire country. It's founder is L. Frank Baum, aka the Royal Historian of Oz and the author and creator of the Oz legacy. It first appeared well over one hundred years ago in Baum's first Oz book titled The Wonderful Wizard of Oz, published in 1900. It is introduced in the eleventh chapter of the novel titled The Wonderful City of Oz, and is often referred to as the "City of Emeralds" in the original text." (The Oz Wiki
The city is built from lime and green terracotta and is empty. The towers are hollow, optimizing customization of the city. I imagine with mods like MrCrayFish's Furniture Mod, you could make the city look awesome. That and the Minecraft Comes Alive or MineColonies mods.