The 1982 Atari 8-bit version of Galaxian, released by Atari, Inc., is authentic enough, and plays pretty much like the arcade original, even if it does look quite basic in comparison. Where it does outdo the arcade version, though, is in the variety of play modes that it offers.

Pressing the Select key (F3 in Altirra) cycles through the ten numbered play modes (0 to 9 – nine being the hardest), plus an eleventh, called “B”. The manual says that this is a special level for Beginners.

Higher difficulty levels mean more dive-bombers and more bullets fired against you. So if you want a true ‘bullet hell’ experience, bump that difficulty up.

More: Galaxian on Wikipedia
More: Galaxian on atarimania.com