First of all, let's get this straight. This level isn't called The Library, but it might as well be. You don't actually reach the structure on Installation 05 that houses the Index, the one that's called The Library by Tartarus and Commander Keyes, until the very end of the next level, Quarantine Zone. However, that level is a mix of indoor and outdoor environments with a bunch of vehicles thrown in and friendly Covenant troops alongside while you battle both the Flood and the installation's defensive systems.

You first see those systems in Sacred Icon: tiny, glowing gossamer versions of Sentinels that flit from place to place, seemingly to try and repair the horrible mess that part of the installation has become; the usual Sentinels and others with more powerful, blue sentinel beams; and the Enforcers, huge floating juggernauts (vaguely resembling the units that shared that name from the Marathon series) that rapidly fire needler-like rounds from the front, a barrage of missiles from the top, and can crush vehicles between two powerful legs if they get close enough.

The stucture of Sacred Icon is both derivative of The Library and indicative of a conscious attempt to address some of the complaints players had about it: that it was dark, repetetive, didn't feature a wide enough array of enemies, and takes place entirely indoors and without any vehicles. Some of these complaints were addressed in the Quarantine Zone level, which as I've said brings the final approach to the Library outside for a bit, with a chance to use the Spectre and the Wraith while watching the Flood perform their new tricks, barrelling at you in Warthogs and Ghosts. (The animations of the Flood driving vehicles while standing upright on the seats are also quite amusing.)

In Halo's Library, you were teleported to the bottom of a shaft that contained the Index and told by your guide, 343 Guilty Spark, that it was necssary to ascend to the top of the shaft to retrieve the Index. The first question any player would naturally ask himself is why GS couldn't just teleport you to the top of the shaft in the first place.

After you've played through the level, winding your way through dark, repetitive corridors with no allies, fighting only Flood combat, carrier and infection forms, with no Covenant and no Marine allies, only the garrulous Guilty Spark and his silent Sentinel cohorts, you'll be left wondering what Bungie was thinking at all; there's almost no variation in the encounters, save for one point where a door fails to open and you're stuck in a narrow space as Flood pour in all around you.

Sacred Icon addresses nearly all of these concerns. Many areas are appropriately dark: it's understandable that Bungie doesn't really want the Flood associated with sweetness and light. Even Two Betrayals, the open-air snow level that plays as Assault on the Control Room but in reverse and with Flood, is noticeably darker than the previous iteration of the same environment. That trend continues in this level.

Unlike the windowless darkness of the Library, there are many locations in Sacred Icon where the structure you're traveling through-- called the Wall-- is open on one side, allowing you to look out over a wasted snowscape that looks like some kind of twisted evil Christmas postcard from a parallel universe. It also allows the level's collection of Sentinels and Enforcers to take aim at the Flood-- and, incidentally, at you and however many of your Grunt and Jackal buddies that manage to survive to back you up.

Where you traveled upwards in the Library, in the Wall you start at the top and must work your way down. The Wall is designed for the travel of Sentinels, and indeed seems to be a place where they are produced; the entire level is strewn with doors where Sentinels will enter. The production of Sentinels is preceded by an audio cue, and you can destroy the entrances so they no longer function.

Although Sentinels are annoying-- unlike the Library level where they were the Chief's allies against the Flood, here there is no love lost between them and the Arbiter-- you may want to consider leaving them alone for a bit at the beginning of each encounter. They will help soften up the Flood in each area, and destroyed Sentinels will drop Sentinel Beam weapons that are at least as effective against Flood as human weapons are, and more effective than most Covenant plasma weapons-- luckily you'll find few of those here, but there are Carbines and Needlers around. You'll also find a healthy supply of human weapons including SMGs, Battle Rifles, pistols and even a few shotguns, although ammo is sometimes scarce.

To travel downwards through the Wall, you need to activate a series of pistons. Sometimes these pistons simply drop through the floor, allowing you entrace to the level below. Other times, they open onto a twisting system of airshafts that lead much deeper and eventually outside the structure. Other times, there are a series of pistons that must be activated; this happens early in the level, when you must do it to activate an object very much like one of the gondolas from Regret that will carry you to another part of the Wall.

There are a number of different rooms and environments in the level, and they are mixed up nicely, with none of them repeating too often. There are narrow corridor areas connected by bridges over dark chasms, galleries that overlook a snowswept vista that stretches to the Library off in the distance, and ribbed connecting tunnels that have the annoying effect of making it difficult to move weapons around on the floor. It's unclear if this is intentional, although Bungie seems to always have had an anti-resource management streak in their game design, emphasizing quick action over methodical thought and urging players to do the best they can with what Bungie decides to give them, when they decide to give it.

In fact, the piston mechanism is one of the most effective ways of achieving this-- much more convincing and somehow less annoying than the way the elevators worked in the Library. Those lifts were effectively one-way. Once you got on a lift and it took you up, you would get off, and the lift would head back down. However, approach the lift and it reverses to come up and meet you, and there was no way to get it to go back down. That was annoying because it broke verisimilitude; there would seem to be little reason in real life to have a one-way elevator. However, once you take one of Sacred Icon's pistons to go down, you can't use them to go back up, but somehow it seems more natural that way.

The level ends with a bit of outdoor action, as you meet up with a group of Elites and support troops in a small encampment just over a bridge past the Wall on the way to the Library. Once you fight off the attacking Flood, there's a cutscene with the Spec Ops commander that starts the following level, Quarantine Zone.

More than any other single level in the game, Sacred Icon is a response to fan reaction to the first game. It's more varied, more interesting to look and and to play than the first game's Library level. About the only thing it doesn't have is the Library's claim to fame: all the bits of background story that were revealed by the Monitor, 343 Guilty Spark, as you traveled through the level-- if you were close enough to hear and could pay attention while you battled the Flood. Of course, Installation 05's monitor doesn't seem to be handy this time... but you find out why later.