Source

Sources of the game can be found on:

License

Free Software

This game is Free Software under the terms of version 3 of the GPL. This means you have four basic freedoms:

Freedom to use the program any way you see fit.
Freedom to study and modify the source code.
Freedom to make backup copies.
Freedom to redistribute it.

The GPL is the legal mechanism that gives you these freedoms. It also protects you from having them taken away: any derivative work based on the program must be under GPLv3 as well.