The five different boss types pose a significant threat on the hardest difficulties.
There's even a story, although it serves little purpose other than to provide some context for the boss fights that pop up every five songs or so. The idea is that a vile entity from beyond the solar system is harvesting the souls of composers and using their music against you, and it's your job to liberate them and your music. That's where the "Symphony of Souls" comes in, and you unlock each of its five parts by battling one of the five bosses that appear in random songs.
As you progress, enemy movements become more complex, new enemies appear, and you unlock six different difficulty levels. The names of the difficulty settings themselves maintain the charm of the whole, as each bears an appropriate name, such as pianissimo, mezzo-piano, and fortissimo. And as a nod to competition, there's a leaderboard feature that lets you chart your prowess on Steam song by song. If you're less into Lady Gaga and Skrillex and more into M. Ward and Steve Earle, you thus have a better chance of dominating your own boards.
Still, Symphony's not without its problems. If you're using an older version of iTunes, you may find yourself unable to play the m4a files that most of Apple's music files come packaged as these days. A quick update of iTunes is usually enough to fix the issue, but the problem persisted on at least one computer even after the update. There are also some minor but stubborn lag issues, which rear their heads when new enemies appear or the tempo changes, even when playing Symphony on the lowest settings. They're never so discordant that they render the game unplayable, but it's frustrating when you lose some of your hard-earned inspiration just because a split-second burp in the frame rate lands an enemy projectile in your paper-thin hull and knocks you out of action. It's also regrettable that developer Empty Clip Studios borrowed so heavily from the look and feel of Geometry Wars; with a unique visual design, Symphony might have distinguished itself from the crowded library of shooters even further.
For all that, though, Symphony is both fun and often addictive, particularly when you experience the wide range of scenarios presented by a massive music library. When it's not bogged down by brief spurts of lag, the mouse-based combat is responsive and fluid. Its unique approach to upgrades gives it enough of a novel edge over its two competitors to warrant some attention, and it's yet another exciting way to interact with your music collection. The worst of its issues could be fixed with a simple patch, and even in its current form, you'll still get more fun out of it than you will from most albums with the same price.