To the game itself:
Very nice! I really liked hovering over those buttons!
The routing of the enemies was nicely done, I saw no bugs, it worked flawlessly
I'd like more content, but I'm sure you know that
To the way you ship it:
Please add some startup scripts. Double-clicking jars doesn't work easily with natives, I'm afraid. But for that reason we have command line scripting:
Add a linux file "startLinux.sh":
java -Djava.library.path="nat/linux" -jar galaga.jar
mac is a little more complicated, afaik... App bundles and such. I have never worked with a mac in my life, so I can't tell you how that works...
And for windows add a file "startWindows.bat":
java -Djava.library.path="nat/windows" -jar galaga.jar
So that would help others trying out your game ^^