Scroll down and download the file for your platform from the Assets section.
Then follow the manual to learn how to use the engine.
In addition to the engine, you also want to install a Pascal compiler and IDE. Like Lazarus and FPC -- so most Windows users want to download Lazarus for Win64, and Castle Game Engine for Win64. Or Delphi.
The bin/ subdirectory contains compiled executables for all our tools, including our Castle Game Engine Editor (castle-editor) which provides an integrated environment to develop games (manage projects, design UI and scenes, build...).
Note: this release is shown on GitHub as released ... 31 Mar 2019 , but don't be fooled by this old date :) This release is actually being constantly updated by our Jenkins, from CGE master branch, after it passes a lot of automatic tests. So this is the very latest and fresh CGE version.