The code was discovered by @memory_fallen, and allows you to make walls in the game semi-transparent.
To access this feature, pause the game and then press: X, Right, B, Y, X, Right, B, Y.
The Saturn port of Doom isn't thought of in especially fond terms by fans of the series, but this is a cool little bonus feature – and the fact that it has taken almost 30 years to find it is pretty remarkable.
