Quote: "Is it possible to make a game compromised of several levels tha can be travelled between?"
AKA Myst, right? Hmm... if you could somehow rig up diferent win zones or something that took you to different levels based on whichever one you walked into or used or whatever, it might be possible.
Quote: "Also can I build underground sections such as tunnels underneath hallways etc?"
Well, there's no set "ground" in FPSC. It really just depends on floors relative to one another. So, yeah - a tunnel under a hallway is very possible.
Look! A monkey!