Sprites suitable (as draft or placeholder art) for oldschool FPS shooters in the spirit of Wolfenstein 3D. Sources that have been used as input:

https://opengameart.org/content/misc-dark-fantasy-scenery-sprites
https://opengameart.org/content/vines
https://opengameart.org/content/dancing-seaweed
https://opengameart.org/content/candles-oil-lamp-and-eye-with-varied-stands
https://opengameart.org/content/barrels-mage-city-arcanos-remix
https://opengameart.org/content/wild-plant
https://opengameart.org/content/pixel-art-keys
https://opengameart.org/content/toxic-barreldrip-animation
https://opengameart.org/content/nature-props-surface-forest
https://opengameart.org/content/rpg-sprite-sheet
https://opengameart.org/content/skeleton-sprite
https://opengameart.org/content/ammo-pack
https://opengameart.org/content/dark-fantasy-item-sprites
https://opengameart.org/content/gold-treasure-icons-16x16
https://opengameart.org/content/flags-0
https://opengameart.org/content/skeleton-and-ghost-spritesheets-ars-notoria
https://opengameart.org/content/bad-kitchen-tileset
https://opengameart.org/content/weapon-inventory-graphics

All of which have been released as CC0. Any other drawing work is my own. Tools used to draw: Gimp and LibreSprite.