In terms of chords and melody, Hudson's Adventure Island II NES - Boss Battle is more complex than the typical song, having above average scores in Chord Complexity, Melodic Complexity, Chord-Melody Tension and Chord Progression Novelty.
Hudson's Adventure Island II NES - Boss Battle is written in the key of A Locrian. According to the Theorytab database, it is the 5th most popular key among Locrian keys and the 77th most popular among all keys. The A Locrian scale is similar to the A Minor scale except that both the 2nd and the 5th notes (B♭ and E♭) are a half step lower. This means that the characteristic "home base" chord of A Locrian has a dissonant diminished quality (a˚), which makes it a very uncommon choice for songs. See the A Locrian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!