T' was said earlier that the 3d effects of this game are not mimicked but are real 3D through openGL. Could you give a general idea how one can add and manipulate 3D graphics on top of a traditional 2D game framework?
It's very very easy.
Step 1: Set up your viewport (either ortho or frustum, basically)
Step 2: Make sure depth testing is off.
Step 3: Draw all your 2D crap (if you're using ortho, just do it using X/Y coordinates, if you're using frustum, rotate it against the camera)
Step 4: Turn depth testing on.
Step 5: Draw all your 3D crap.
Getting stuff to line up just right can be a slightly difficulty but (especially if you're using ortho) it really isn't very hard at all.