I need simple collision detection.

The user is limited to a certain amount of squares that they are allowed to walk on. If they go on to a non-walkonable square, such as...oh I don't know, a 120 foot tall solid steel chicken, they stop as if it were 3D.

Does anyone have even ONE way to do this.

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"I'm carrying a Geometry book, but I'm not in Geometry...it's crazy...crazy man!"