Radiant Silvergun, that masterpiece of design and engineering, is surely the most famous Japanese shoot 'em up from the genre's golden age. But it's Battle Garegga that, for many, holds the era's crown. Released in arcades in 1996 it's by far the plainer-looking creation, a muted palette of World War II gunmetal greys and khakis foreshadowing the next decade of militaristic video game colour schemes. Unlike its peers and rivals, which have your fidgety aircraft feinting and swerving through a brightly coloured hail of enemy bullets, Battle Garegga's designers make you navigate a shifting maze of realistic looking artillery fire. The grey and silver bullets blend infuriatingly with the tops of trees and froth seas over which they zip. No boss fights with sprinting deities here, either. Just a corridor extending up the screen and beyond, filled with battalions of crotchety tanks, kamikaze biplanes, and the odd spindly air fortress.

By 1998's launch of the Sega Saturn version (the only one to successfully flee the arcades till the unbelievable arrival, next month, of a twentieth anniversary PS4 rerelease) there was little to mark Battle Garegga out from the shmup pack, aside from the prestigious name of its developer, Raizing, and the unlikely choice of publisher, EA. Sure, this was a game filled with a bewildering cascade of tokens and power-ups, but at a glance, its rules seemed routine, obvious, antique: Shoot them before they shoot you; Collect everything in sight; Make it to the end in fewer than three lives. Then, slowly, players began to notice arcane complexities.

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For example, up to four drone-like 'options' could be collected and added to your ship's arsenal. With a press of a button these could be arranged into different configurations, providing a wide spread of fire across the screen, or made to spin in a circle around your craft. Canny players noticed that, if you collected medals in a certain order, new configurations for your drones would be opened up. Then, there was the question of the game's weird difficulty. Players who followed the typical shoot 'em up play style, taking down every enemy, collecting everything in sight and so on, would find that by the time they reached the game's latter stages the game had become almost impossibly challenging. Conversely, players who died frequently along the way had a far easier time of it. Battle Garegga was unquestionably a classic. But nothing quite made sense in its knotted, topsy-turvy world.

A number of players, infuriated by the obfuscation, took it upon themselves to reverse engineer the game's ROM, picking through the lines of code for clues as to how it all worked. What they uncovered was one of the deepest and most unusual systems ever conceived in an arcade game, filled with as many swerves and feints as the aircraft that flew through it. Buckle up, because this will be worth the ride:

Many shmups have a foundational mechanic known as 'Rank Up' whereby the game's difficulty dynamically adapts to the player's skill. In Gradius, for example, every time you gobble up a power-up and Vic Viper increases in speed or power, every enemy in the game becomes a little stronger and quicker in kind. It's a way to ensure that a good player doesn't simply blaze through the game in a single credit, and that a weak player isn't obliterated by challenge. The system is typically hidden because, if it were to be revealed, it would undermine a player's sense of power and progression. What's the point in upgrading your weapon if everyone else's gun weapons level up at the exact same moment? In essence, Rank Up means that a game's challenge remains constant and calibrated, regardless of a player's talent. And in this way, the game remains profitable for its operator.

A hidden English language menu in the Saturn version allows you to toggle an option to play as four of the ships from Raizing's wonderful racing shmup curio, Kingdom Grandprix.

Battle Garegga doesn't quite expose its deep and mystical Rank Up system, but it does introduce it into the meta-game in a way that no other game has done before or since. The reverse engineers discovered that almost everything you do in the game contributes to a 'Rank Up' counter. Every shot that you fire, every yard that you fly, every enemy that you down is given a numerical cost that reels up and up and up. For every seven frames of gameplay, for example, 22 'points' are added to the invisible gauge. The moment you ship takes off, the world around you is becoming hardier and more hostile. Enemies become more aggressive; they fire at a quicker rate, their bullets travel with greater velocity, and in tighter formations of buckshot. Foes become more resilient. The items that they drop tumble down the screen at greater speed, making them harder to catch. Grab everything and shoot everyone without dying and the game Ranks Up so quickly that, by the time you each its fifth stage, nobody could survive.

Rank Up must be mastered, then. You must constantly monitor your acquisitions, ensuring that you don't become too powerful too quickly, or fire redundant shots up the screen when there's no target in sight. It's a game that encourages you to never be wasteful or inconsiderate. Moreover, it's a game that encourages you to actively force the Rank down again, by committing what's referred to by the game's players rather brilliantly as "thoughtful suicides". By ramming your craft into a bullet or a wall at key points, the game's difficulty is forced down, making progress manageable again. The game enjoys one of the most expansive and detailed internet guides of any.

Some see Battle Garegga's twists and curiosities as needless complication. And certainly, any player's first few runs will feel bewildering, almost oppressively intricate. This is virtuoso game design within its chosen niche and as such it's rich and heady, best enjoyed in short doses. But in time Battle Garegga becomes an extraordinarily rewarding pursuit, encouraging you to consider the game from rare and unlikely angles. During the past decade it has become one of the Sega Saturn's most sought after and, as a result, expensive games. In December M2, those masters of the re-master, launch a digital update of the game titled Battle Garegga Rev.2016 under the company's new 'Shottriggers' label. The update will make some of the original's complexities easier to understand but, more importantly, will make the game available to every PS4 player. For all its routine, slightly drab looks, this is a game of exquisite artistic intention. Long live the mad king of shmups.