From OpenClonk

You can download and test the latest automatically created development snapshot. A development snapshot is a zip file which contains everything to run the game on Windows, i.e. an engine binary and all game content. Development snapshots are updated once every week and are meant for players or those who want to try out OpenClonk the easy way. This build should run out of the box, if it doesn't, post the error you get in bugtracker.

Or, you can grab a full copy from the repository using Mercurial. See our Developers Guide.