Install castle-game-engine-doc by entering the following commands in the terminal:
sudo apt update sudo apt install castle-game-engine-doc
Castle Game Engine - Developer's Documentation
Castle Game Engine is a set of LGPL licenced libraries that are intended to ease developing 3D games with FreePascal / Lazarus. . It provides an excellent support for the VRML / X3D 3D data format. Other 3D formats are also supported. . It features many advanced graphic effects and easy to use API on top of OpenGL. . This package contains documentation.