Space Invaders '91

Space Invaders '91 is Space Invaders with a health bar, some more annoying enemies, and a company that paid attention to what the current year is (see right).

Round and Sound Select
The game has both a secret Round Select and a secret Sound Select (sound test) that run on the same code and work the same way.
 * Round Select: press A + C + Start on the title screen, then B, A, C.
 * Sound Select: press A + B + Start on the title screen, then Right, Right, Down.

Up and Down scroll; Start selects.

Regional Differences
The enemy layouts in the two sections of the final round, Doriasta, come together to form the phrase "インベーダー９０" ("Invader 90", part of the game's Japanese title; the ー after the ベ is underneath for some reason) in the Japanese version and "INVADERS" in the US one.

This in turn leads to the two versions having different amounts of enemies: 32 and 40 respectively in the first section, 42 and 41 respectively in the second.