Unless otherwise noted at a particular page, the content (text and images) of the https://castle-engine.io/ website is:
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Copyright by Michalis Kamburelis.
Moreover, some graphics is also copyright by Paweł Wojciechowicz from Cat-astrophe Games. This concerns the Spine Dragon (also available in https://opengameart.org/content/cge-dragon-animated-in-spine https://github.com/castle-engine/castle-engine/tree/master/examples/2d_dragon_spine_game ) and CGE logo (see https://github.com/castle-engine/castle-engine/tree/master/doc/pasdoc/logo ).
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Covered by a number of open-source licenses (you can use any of the mentioned licenses to redistribute):
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GNU General Public License (version 2 or later, at your option).
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Creative Commons Attribution-ShareAlike 3.0 Unported License (CC BY-SA) (same as one of Wikipedia licenses)
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) (newer version of the above license).
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GNU Free Documentation License (GFDL) (unversioned, with no invariant sections, front-cover texts, or back-cover texts) (same as one of Wikipedia licenses).
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This copyright and license doesn't apply to any user-generated content (e.g. blog comments, forum posts and so on). That remains the property of a user that created it.
Except for the above (contents of the website), the rest of the files inside the repository https://github.com/castle-engine/cge-www are copyright by Michalis Kamburelis and can be reused on terms of GNU General Public License (version 2 or later, at your option).
This concerns stuff like
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Shell scripts to pack releases
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PHP code (not visible to user of the website)
In other words, these things are not covered by Creative Commons or GFDL licenses. They are only covered by GNU GPL license.