Having had success with the sequel to Galaxian's arcade version, Namco had decided that after the NES had appeared to have established a solid market for itself, it should make a straight port of Galaga like it did Galaxian; and like Galaxian, the NES port came many years after the arcade version had debuted.
Again, the result would be comfortably but also disappointingly familiar to those who had played the arcade version long before the NES version.
The premise is again about a lone ship with comparatively lousy mobility that has to battle much more nimble and faster alien enemies; this is unchanged in the NES version.
Galaga's aliens looked even more like space bugs than Galaxian's, which was somewhat an aesthetic upgrade over the first game. The NES did provide some enhancement over the arcade version, such as more frames for the sprites and additional shading, but these are just cosmetic upgrades.
As in the arcade version, Galaga for the NES had the alien enemies zipping into the screen at very fast speeds, but often in patterns that have less purpose than they did in Galaxian. This makes them a lot more unpredictable, but also more entertaining to watch, especially if the player can blow them up before they reform in the familiar formation of the alien armadas that were prevalent in Galaxian.
However, the player can always expect some of them to detach to make a fly-by at the player's ship while firing some shots, whether they are already in a formation or are still flying in patterns after spawning into the screen. They detach even more frequently than they did in Galaxian, making them even more troublesome than ever, especially now that they move more quickly.
Taking down aliens efficiently, i.e. having every hit striking an alien ship (which always go down in one shot, like any of the player's ships), has the game awarding bonus points to the tune of brief upbeat music.
Sometimes, the elite of the alien swarms, which can take two hits each, may come over to emit a tractor-beam-like wave that can pull in the player's ship, effectively capturing it and forcing the player to expend a life to bring in another ship. However, the captured ship remains on-screen, linked to the elite alien that captured it.
The player can attempt to rescue the captured ship by shooting out the captor, though there is a risk that the player will hit the captured ship too. However, if the player succeeds, the captured ship does some twirling animation (during which it is still vulnerable to being accidentally shot) before joining with the player's ship, effectively doubling the player's firepower but also doubling the sprite size of his/her player character (though a shot that hit either ship only takes out that one).
Every few levels or so, there will be a bonus level of sort that lets the player fire on formations of aliens that fly into and out of the screen, simply begging to be shot down for the player to gain bonus points with. Reaching score thresholds awards extra ships, as to be expected from a score-oriented game.
The aforementioned mechanics were what made Galaga a lot more entertaining than Galaxian ever was, and also somewhat easier to play.
Yet, these designs were already in the arcade version; the NES version added nothing truly new, which could have been a bit disappointing.
In conclusion, akin to the NES version of Galaxian, the NES version of Galaga would have better value for those who had yet to play the arcade version; to other players, it would seem like Namco was just squeezing a bit more income out of the game to take advantage of the market that the NES had.