At various points you'll be supported by Idols, which can house one of the ghosts Poe carries around. Throw the Mercy ghost into an Idol, for instance, and it'll heal you. On the other hand, the Mercy ghost makes the game "easier" and so using it will incur a penalty to your score multiplier. Other ghosts will fire at enemies or freeze them in place, and you'll unlock more as you progress through the levels.
It's a bunch of disparate systems, but they're all tied together by what Harmonix does best. Remember when I said this was a rhythm game?
It starts with the backing track, which acts as the foundation for the level. From it, you'll be able to discern the basic rhythm of the level, be able to sense when the action is about to get more intense, et cetera.
The next layer is Poe. Poe's weapon isn't just for show. Each shot you fire triggers percussion noises, whether it's the sweep of brushes across a snare drum or what sounds like two metal rods clanging together. Stop firing and the rhythm section also stops.
When the rhythm speeds up, Poe's shots speed up. When the rhythm slows down, her shots are correspondingly slow. This leaves you subconsciously listening to the backing track for clues as to how Poe will control at any given time, especially when it comes to the lengthy (and difficult) boss battles. You could play A City Sleeps with the sound off, or without paying attention to the music, but you're probably doing yourself a disservice.
Luckily, enemies play by the same rules. Enemies appear in bursts of sound, live to add to the soundtrack, and die with another burst. Chimes, synths — enemies are the third layer and provide much of the music's richness. Seeing the massive onslaught of projectiles a boss releases as a result of a saw synth is impressive, but you're just as likely to die from a hellish cloud of bullets made up of tiny, non-threatening bell noises.
Idols add a fourth layer, when present, and one that changes based on whichever ghost is in play. Of course, the music is also important for cluing in when the Idols will activate — for instance, knowing when you should be on top of the Mercy-activated Idol to receive a burst of healing, and then flying away before enemy bullets can rip through you.
The way Harmonix ties the music into the visual patterns is what makes this a gorgeous and fulfilling experience. It's a lot like playing Soundodger but with more thematic consistency, or like playing a more polished and streamlined version of Beatbuddy.
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