Here is a level cheat for IWAR:

            Open the IWAR directory on your hard drive. Inside there should be a
            folder named "PSG". Open it. Inside of the PSG folder there is a
            folder named "RESOURCE". Open this too. Inside of the "RESOURCE"
            folder there is a folder named "SCRIPTS". Inside of "SCRIPTS" there
            should be a folder named "PRECEDENCE". Inside of the "PRECEDENCE"
            folder there should be a file named "PREC" which you should open
            with a spreadsheet program, like Excel, and change all the numbers
            in the "PREC" column to 0, and save. Now you can access all the
            levels of IWAR from the beginning.