Clonk game content is available under the following license:

Creative Commons Attribution-NonCommercial (CC BY-NC)
http://creativecommons.org/licenses/by-nc/4.0/

This applies to graphics, audio data, scripting, and text found in the
game release packages (usually packed inside c4f, c4g, and c4d group files).

You are free to:
    Share — copy and redistribute the material in any medium or format
    Adapt — remix, transform, and build upon the material

Under the following terms:
    Attribution — You must give appropriate credit, provide a link to the
    license, and indicate if changes were made. You may do so in any reasonable
    manner, but not in any way that suggests the licensor endorses you or your use.
    NonCommercial — You may not use the material for commercial purposes.