To be able to reach this hidden stage select in Journey to Silius a Game Genie is requires. Enter the code XTUSKTAV
and you'll be directed to a stage select screen after pressing start on the titlescreen. By pressing button A or B you
can choose the stage you want to play. When start is pressen the game will begin at the chosen stage.
If you use the code above combines with this one - TEXINTIA - You can go directly to the final boss, but make sure you
choose "Final Stage" on the stage select screen.
But that's not all...
If you press B 33 times while on the title screen and then press start, you'll be taken to a sound test menu. Use A or B to select the tune you want to hear and then START to play it.
Also, in this menu you can enable more continues.
On the title screen, press UP, DOWN, A, B, and then START to activate the Doom Gun.