Scroll down and download the file for your platform from the Assets section.
Then follow the manual to learn how to use the engine.
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: Ignore the date of this release shown by GitHub, which is far in the past now (31 Mar 2019). This snapshot release is being constantly updated, after it passes automatic tests, based on the commits to the "master" branch. The files available to download here always reflect the very latest engine version.