@Ulrich Metzner

These assets were very helpful when we came to create our game. I understand that these assets are public domain, however if you would like to be credited please let me know how you would like to be addressed (screen name, full name etc). A link back to this site will be provided if no other information is submitted.

Thank you again

@Sakdiwaa

These are not Public Domain.  Licencing shows CC BY-SA 3.0 which requires the following:

• 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.
• ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.

Source
https://creativecommons.org/licenses/by-sa/3.0/