Collect all 30 Elephant Statues to unlock new costumes for all four characters.
Goemon GBA Link-Up Bonus
Turn off your DS and put Kessakusen! Ganbare Goemon 1+2 for Game Boy Advance into the GBA slot and the DS Goemon game into the DS slot. Turn on your DS and start the DS Goemon game. The GBA game acts as a dongle that unlocks new characters: NPCs from the GBA titles appear throughout the DS adventure and Space Mambo: This sidescrolling shooter is accessible from the mini-game menu as long as the GBA cart is inserted.
Goemon New Age Shutsudou! reference
In the second town's underground prison, New Aged Goemon can be found occupying the second jail cell for his "crimes against the Goemon franchise."