Princess sprite is from Jason-Em. Source: http://opengameart.org/content/bushly-and-princess-sera
Hero sprite is from Arachne. Source: https://forums.tigsource.com/index.php?topic=9147.0
Fireball sprite is from Matheus Carvalho. Source: http://opengameart.org/content/fireball-vector
Monster sprite is from dogchicken. Source: http://opengameart.org/content/cute-monster-sprite-sheet 
