Source link is broken in website...
That's not our fault, it's due to an attack against Sourceforge: https://sourceforge.net/blog/
They say that all services (including svn source-downloads) will be back again this week. Until the you may download the sourcecode directly via SVN. Make sure to have subversion installed and type in your terminal:
svn co https://mars-game.svn.sourceforge.net/svnroot/mars-game mars-game