Every time you beat the game, any new game started will have its difficulty increased. The level is measured by crystals that appear on the main screen. Every game cleared will add another crystal, and ten small crystals will combine into one large crystal. The difficulty is maxed out at 5 big crystals, which is 50 playthroughs, each harder than the last!
Save the game at the last save point. The Seraphic Gate will now appear on the world map. After beating it once, you can run through it again with stronger foes.
Original Valkyrie Profile battle music:
Beat the game and load the cleared file. Hold L1 when entering a battle to hear the original game's battle music.
Random battle music:
Beat the game and load the cleared file. Hold L2 when entering battle for a random song to play.
Thanks to Revolution Readers Halar, Chuck and Buki!