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This is a puzzle game, really: the fastest, most intense puzzle you'll ever play.
Sure, Treasure's spiritual sequel to Saturn classic Radiant Silvergun might feature plenty of shooting, but it was a bold step into more cerebral territory for the vertical scroller - its polarity-shifting as crucial as where you're pointing your twin guns.
When your craft is white, you can absorb and fire white bullets that double-damage black enemies but do little to white ones. And when you flip to black... well, you can guess.
Absorb enough projectiles and you can fire a homing laser best saved for larger enemies and bosses - or any other time you might be in a pinch. In Ikaruga, that's most of the time. It may only be five levels long, but actually reaching the final stage - let alone beating it - is a challenge that many will fail.
Yet despite the fact it's an incredibly demanding game (one that will only truly be mastered by a tiny portion of an already niche audience), Ikaruga is never anything less than entirely fair. Indeed, there are generally two approaches in any given situation: adopt the same hue as the predominant enemy for a safer way forward, or switch for a potentially quicker but more hazardous path.
Of course, that's before you consider the combo system. Scores multiply with every three like-coloured kills. Simple in theory but exacting to pull off under the pressure of a hundred death-dealing dots.
It may be a simpler game than its predecessor, and shorter too, but it demonstrates such confidence in the purity of its design - it knows it doesn't really need anything more. Ikaruga was made chiefly by just three people over two years, which goes some way to explaining both its wonderful single-mindedness and its rare level of polish.
It's also beautiful, as elegant and striking as the Japanese Grosbeak bird from whose native name its title is taken. Little wonder many gamers tried flipping their TVs through 90 degrees to lose the borders framing the action; a fitting course of action for a game designed to break boundaries.