Controls were all inverted (up was down right was left) until I turned "Invert controls" on.
It didn't really feel like zombie spawns had much relation to either the melody or the rhythm of the song I tried. While the spawns were on-beat it didn't feel like there was much relation between the song and what was going on in the game.
Another thing I noticed is that Easy is felt harder than Normal. The low speed of the zombies causes them to spawn in piles on top of each other which makes the timing harder to get.