Does the 3d engine use the same underlying code as the 2d?

Well, the question is in the topic header, really.

I am just interested in getting confirmed if the 3d is like a "skin" on top of the code - a new view layer - instead of the 2d.

The reason I am interested is, if it is so, then the game will be as polished when using the 3d as when using 2d.

EDIT: Well, I just read in the long info-thread that "2D and 3D are just different ways of viewing the same engine, so from that side of things working on the AI is working on both.", so I guess that answers my question. Feel free to delete this thread. :-)