Like many bullet hell games, Ikaruga was originally released in arcades. It made its console début on the Dreamcast, then later ported to the Gamecube, then finally brought to the Xbox Live Arcade.
Ikaruga has an interesting 'polarity' mechanic. Your ship can flip between light and dark polarities which gives you a different coloured laser and shield; blue or red respectively. This duality means you absorb any enemy fire that matches the colour of your shield, otherwise; you are destroyed. Your attacks will damage enemies regardless of their polarity, but opposite polarity deals twice the amount of damage compared to the matching polarity. Initially, when bullets flood the screen, your natural gaming instinct is to avoid them regardless of colour. It takes a while to train your mind to ignore the opposite colour bullets, and requires a high level of concentration as you switch between polarities.
Switching polarities is almost instantaneous, so it gives you enough flexibility to switch at the last opportunity, but you cannot spam the maneuver to stay safe. Throughout the game, all sorts of patterns of bullets are thrown your way, so you are required to weave and switch polarity in order to navigate the danger. Earlier on, you may just have to dodge a large number of bullets, but later on, there will be nets of intersecting lasers that you can only get through by timed switches of polarity.
The bullets you absorb fill up a meter, comprised of twelve boxes. By pressing R, your ship releases a set of missiles that home-in on the nearest enemies. The number of missiles is proportional to how full the meter was.
At the end of a stage, there is a boss. These boss battles are timed, so either end when you defeat the boss or run out of time (and the boss flees). Defeating the boss awards you with lots of points, but you don't score anything if the boss flees.
Surviving and maximising your points comes from practice and memorisation of the stages. If you destroy three like-coloured enemies in a row, you are awarded with bonus points and the opportunity to start a 'chain'. If you carry on this way, your bonus multiplier will increase as the chain grows. To take advantage of this, you need to shoot carefully, which then makes your task of surviving more difficult. Chaining makes a huge difference in score which gives you extra lives as a reward (and bragging rights!). At the end of the level, you are given a performance grade (C-S++) based on your total number of points, if you destroyed the boss in time, and if you used continues.
Like most bullet hell games, Ikaruga is incredibly short at only five chapters long. However, the level of difficulty means there's a pre-requisite of hours of practice, honing your skills on the individual levels before completing a successful run. The length of the game is perfect for the difficulty because you feel like you haven't lost a lot of progress when you die, often leading to that 'one more go' attitude. It's enjoyable to replay the levels because you should notice an improvement each time you play. Extra continues are awarded every hour of play-time, so you should complete it with enough perseverance.
There are few difficulty levels. On Easy, enemies do not send parting shots when defeated. On Normal, enemies fire more bullets and send parting shots if you are the same polarity as them. Hard brings more bullets and parting shots regardless of your polarity. Obviously, if you stay the same polarity, you will merely absorb these bullets, adding to your power, but switching polarity means they pose a danger. Since you'll be switching polarities frequently, you'll have to be aware of the current on-screen bullets and take into consideration bullets from exploding enemies.
When you do complete the game, you unlock Prototype mode. In this mode, you have a supply of ammunition which can be topped up by absorbing bullets. This means you cannot just permanently hold down the attack button like you can in the main mode.
Overall, Ikaruga is a joy to play. It features a great game-play mechanic and utilises it perfectly. The levels are well designed which means it maintains a high level of longevity despite its natural brevity.