Monsters from Quaternius
  https://quaternius.com/packs/ultimatemonsters.html
  license: public domain
  support the artist: https://www.patreon.com/quaternius

City buildings and UI images from Kenney
  https://kenney.nl/assets/game-icons-expansion
  https://kenney.nl/assets/city-kit-commercial
  license: public domain
  support the artist: https://www.patreon.com/kenney