Ninja Adventure — Pixel-Boy and AAA. Creative Commons Zero (CC0).
Author and license: https://pixel-boy.itch.io/ninja-adventure-asset-pack
Original tileset bytes retained. Arena palette adaptation and damage overlays performed at runtime.
Source mirror: https://github.com/Papyszoo/CC0-Public-Domain-Sprites/blob/main/packs/pixelboy-ninja-adventure/sprites/background-elements_tileset.png