A True Treasure for the fans of the old-skool shooter.
Gameplay is very simple to get into, but you'll quickly find it's hard to master. Your controls are basically made up of your primary shoot button, your polarity switch (more on that later) and your super-shot release (coming up). Enemies in this game are made up of two polarities: white/blue and black/red, with matching shots. If your polarity (shield around your ship) is the same color, the shield absorbs the shot, you take no damage, and it charges up your super-shot. Likewise, if you shoot an enemy with a shot of opposing polarity (i.e. a white/blue target with a black/red shot) then it does double damage. This imposes a powerful level of strategy, particularly in higher difficulty levels when your enemies start having the habit of doing a "suicide spray" of bullets towards you when they die.
Obviously, this game is about attaining a high score... but even that is an artful thing to do. In order to attain best scores in the game, you have to not only shoot every enemy you can, but do so in chains of three. For every three consecutive kills of enemies of the same polarity, the chain goes up. You can do 3 whites then three blacks, but it has to be three of the same color in succession. If you can keep the chain up, your score multiplier goes stupid-high, and you'll be able to earn high-grades for all levels. So even then there's a bit more thought than simply shooting everything in sight. But additional strategy can then be achieved with the second player, because then you could have one player covering with their shield while another takes care of shooting stuff in opposite polarity (earning big damage and taking care of bosses quickly). All this sounds easy enough, but unless you have a fantastic memory, you'll be hard-pressed to remember every possible enemy combination, and that's where the replay kicks in.
Fair warning to those who haven't played these types of games before: it is HARD. But thankfully, not impossible... and certainly not nearly as "cheap" as games like Ninja Gaiden, Contra, and the like. It's the kind of hard that you'll be motivated to actually try and figure out how to overcome and actually have fairly good luck in achieving, unlike another game where you could spend an hour-plus fighting the same end-boss of a level *cough* ninja gaiden *cough*. At no time does the game bring you to a level that makes you feel like it can't be done... only to a level where you simply need more "credits" to make it further and continue forward.
It'll be a bit hard finding this game now... but by everything held sacred in humanity, is it ever so worth it for you to find this game.