Note that you only have one chance to beat Chester Field. If you die the game displays "Game Over" and returns you to the title screen. Because of this, they implemented passwords to help you skip to a previously achieved point in the game.
Life
Life is the amount of health you have left. It is indicated by the small heart with an "L" inside at the top left of the screen. You can acquire additional health by either picking up the power-ups dropped by enemies (hearts with "L"'s inside) or by spending the night at an inn.
Metal
Metal is the currency used in the game. You can acquire metal by killing enemies and picking up the bags labeled with an "M". The amount of metal that you currently have appears at the top left of the screen next to the oval with an "M" inside.
Combat
When fighting enemies, you can either attack straight forward while standing, attack straight forward while ducking, or attack straight up while standing. You can also attack straight forward or straight up while jumping (you cannot duck while jumping).
If you are not attacking, you are on defense. When defending, your shield will block projectiles. Although this can help you avoid taking damage from enemy shots, enemies will hurt you if they touch you.