Cities underground

Hi fellow modders.
As I wanted to refresh the forums with some stuff I started making a map, which I will upload soon here when it's finished.
While doing so, I came across an idea. As you added undergrounds to the mod, it created a logically invalid situation where a city is underground and clearly sky is visible in castle. How about making a Background that would replace any of the skies in castles to something like this instead of a normal sky:


The other idea is to make new castle (inferno) underground.

It is not a bad idea, but it would be something to be done way down the line as we still got a lot of more basic stuff which needs to be done first.