Rhythm Heaven - Shoot Em Up is written in the key of A Minor.
According to the Theorytab database, it is the most popular key among Minor keys and the 7th most popular among all keys. Minor keys, along with major keys, are a common choice for popular music. The three most important chords, built off the 1st, 4th and 5th scale degrees are all minor chords (A minor, D minor, and E minor).
See the A Minor Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!