As for the original DOS version, I couldn't really find any "websites" with downloads, just FTPs. Really, just search the net for "galac15.zip" if you want to get a copy. I still recommend trying the Windows version at least once.
One thing I didn't talk about were the asteroids. They only show up for a couple sets of waves. You can't shoot them down and ramming into them is obviously a bad idea so just avoid them when you see them.
There's another control bug with the DOS game where firing missiles with a mouse button tends to fire two at once, which is a part of the reason why I didn't use the missiles much.
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