2D world depth levels/layers

Need guidance on topdown/isometric/orthographic view and depth levels

Me and a few of my friends are on the planning phases of making a topdown/orthographic/isometric rpg, and we were discussing an open world much like Ultima and with depth in the world similar to Tibia or Dwarf Fortress. As in, world has multiple layers of depth even tho the game is in 2D. When you enter a tile/block that descend you underground, the character would seamlessly go a layer lower in the world. As in 3 dimensional world in 2D without loading screens.

Are there any guides or tutorials on making a game world this way in unity? I've tried googling high and low but all i get is sprite sorting layers and such.

Thanks to anyone in advance.