To get to the secret level "Grave Yard" you must follow these steps. First go to the Cathedral (level 6)  and flip the end switch. Don't beat the level instead go back up the green key stairs and turn right. Go down the hall until you see an opening in the wall that wasn't there before. Go in there and kill the badguys and flip the switch to open another door down there. Then hopefully you'll go on the right "end" teleport. You'll beat the level then will be sent  to the Grave Yard.

Enter these codes with the keyboard while playing.