Suggestion: sewer system inside roads #732
Description
This doesn't entirely suggest to make a code for a sewer system (although having some system of connect points inside the city and between cities like metro or mentioned sewer system would be cool) but rather a deeper customization of streets. The levels should stay the same but additional layers (cellars) could be put inside top make for example sewers on the streets.
I think it would be easier as streets already tend to rotate. They should be accessible (iron trapdoor at the surface + ladder) and you should be able to walk through them. Handy in many situations. Some may lead even to the buildings, although that may be a complicated thing to code.