Error in CIV2: “Could not load the game.File format is Obsolete”

I'm running Windows 7 Home Premium x64 and I have had no problems using it – I can play games, do some editing – and so far have not experienced any crashing. I tried to open a particular stage to play in CIV2 and clicked on it, but an error popped up which says, "Could not load the game: the file format is obsolete." Please help.
