2D Boundaries in 3D?

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Hello fellow programmers..
I am creating a 3rd person game in wich the 3d-player moves across a 2d-background (Grim Fandango, Monkey Island 4 etc.). I've created an editor that allows one to place lines where the character is suppose to stop from walking too far into the image. However, I can't seem to find a good way of setting up these boundaries in the game. Can anyone give me a hint?