I initially wanted to make an underground medieval city but opted not to. Making a cave system big enough to be good looking requires an insane amount of plots, especially if you are going to make a big city. With the plot system that we have, I already see myself spending so many plots just to keep trees from respawning, I could never see myself build large scale underground sadly.
But if you ever wanna talk medieval ideas with me, I am definitely all for it! I have a medium sized medieval town in the works and love to share ideas and such with other avid medieval builders!
Here is one screenshot from the screenshot post.