To learn the engine, our most important documents are:
The engine uses a cross-platform Object Pascal language and tools (FPC, Lazarus).
If you'd like to learn the Object Pascal:
Resources about the engine from the Web3d 2015 conference workshop:
More documentation about the engine:
Copyright Michalis Kamburelis and Paweł Wojciechowicz from Cat-astrophe Games.
This documentation itself is also open-source,
you can redistribute it on terms of the
GNU General Public License.
Like every other frickin' website on the Internet.