Completely subjective list of places with good quality assets (graphics, sounds) in formats useful with Castle Game Engine. See and post on the #assets channel of our Discord for more.

Quaternius

Many 3D model packs, on a public domain license.

OpenGameArt.org

Game art (3D, 2D, sounds) uploaded by various artists on clear open-source licenses.

Sketchfab

3D models, in glTF format, uploaded by various artists. Some free (on clear licenses, like Creative Commons), some paid.

Note: you can also import from Sketchfab directly in CGE editor!

Kenney

Sprites, 3D models, sound effects on a public domain ("do what you want") license.

Blend Swap

Models made using Blender (can be easily exported e.g. to glTF to use in our engine). On various Creative Commons licenses.

Assets (3D and 2D Graphics, Sound)