Is it possible to make a RTS with more than one floor?

Is it possible to make a 3D Real Time Strategy game? For example, units being able to climb on top of buildings, or units being able to walk over and underneath a bridge. Basically adding a the zed axis to pathing, instead of the traditional 2D pathing which all RTS games have.

I thought it sounded very straightforward, but I am having a hard time finding a game or discussion featuring multiple level / pathing layers / floors.

 

I can't see why it can't be done, you'd just want to be careful about having things hidden e.g. Placing units under a bridge so they can't be seen later when you can't remember where you sent them. Line of sight & angles of fire up & down would come into play as well I'm guessing.

 

Tiberian Sun did this in 1999, so it's definitely possible.

 

Total annihilation sort of did in that you had flying units that flew over ground & sea units, some of which could attack each other depending on weapons, & then there were submarines that went under the ships & could shoot them.

 

Definitely possible, but the challenge would be in making it fun. It just sounds like a game design nightmare. How would you implement the visualization such that it communicates the level clearly to the user, yet provide the challenge of line of sight and fog of war visibility? I would hate to be the designer responsible for it.