File:
  textures/space_*
License: CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/)
Attribution: kurtk84 (available from https://opengameart.org/content/space-skybox-1)
Note: edited with gimp

File:
  sounds/cannon.wav
License: CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/)
Attribution: Lambich (available from: https://freesound.org/people/Lambich/sounds/350579/)