Scroll down and download the file for your platform from the Assets section. To build applications using the engine, you should also download Lazarus (which includes FPC) for the same platform. E.g. most Windows users want to download Lazarus for Win64, and Castle Game Engine for Win64.
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...).
If you upgrade from CGE 6.4, take a look at the "breaking changes" list.