The game was initially released as unlimited shareware.
Later the rights to the source code were transfered to Sam Lantinga and released under the GNU General Public License (GPL) as free software. The original game contents were still non-free although they were free for private use. Besides there are game contents of unknown license status available in the form of mods from third party developers.
In 2010 also the original game contents were put under a free license – the Creative Commons license Attribution 3.0. This makes the whole game available as free software and consequently enables everybody to freely use, distribute and modify the whole game.