Sega Master System / Mark III / Game Gear
When the "SEGA MARK III" appears, hold on controller 1 and on controller 2 until the sword appears in the opening screen. Now, when you start the game, your character will be invincible.
Normally, when you use the continue option, your character appears at the start of the stage. With this code, you'll start at the check-point of the stage. Just select the Continue option while holding on both controllers 1 and 2.
See the main page for Argos no Juujiken (アルゴスの十字剣)