Taito's Space Invaders, which established the fixed-shooter genre, was a great success such that Namco (which is now Namco Bandai) made their own game to get a piece of the pie.
Galaxian may seem like a copy-cat of Space Invaders at first glance, but its designs turned out to give it an identity and success of its own.
Unfortunately, Namco thought Galaxian's designs were so good that they did not warrant much of an update when it transitioned over from its original platform in the arcades, during 1979, to the NES at 1984. The result was a somewhat dated game that did not make much use of the NES's capability at the time, as well as a few minor but irritating glitches.
Like the arcade version, Galaxian for the NES uses the same core formula of a protagonist which resembles a paddle-like entity that shoots at targets that can move more freely. Even the premise remained the same; a single lone ship has to defend against the non-stop encroachment of alien armadas.
The armadas still behave in a manner that made the original Galaxian different from Space Invaders. Where Space Invaders was more forgiving, e.g. providing barricades for the player's ship to hide behind and an idiotic alien armada that just shuffle from right to left and left to right, Galaxian was relatively more brutal.
The targets that the player has to shoot down may initially shuffle from left to right too, but they soon detach from their fleet formation to dive-bomb towards the player's ship while firing shots of their own that are far more maneuverable than the protagonist's straight-firing munitions.
The permutations of detachments remain quite random: various alien ships from different ranks of the formation may detach, and depending on their colour, they may make passes at the player's ship or fire salvos with different amounts of shots. The elites of the armada, which hang back at the rear formations, may occasionally move out with some escorts to make an attack run that can be quite troublesome if they are not taken out quickly.
They generally still die with just one shot from the player's ship, though that also occurs for the latter (still).
Eliminating ships grants points to the player. Removing an entire armada will bring on the next one in just a couple of seconds, making sure that there will be no rest for the player, even if he/she can rack up high enough scores to breach thresholds that grant extra lives.
Yet, if a player had played the original Galaxian at the arcades, he/she would notice that there had not been much of any changes since the arcade version. The tune that plays at the start of every playthrough was even very much the same that played in the arcade version.
The wholesale transfer was not limited to the above: the sprite for the player's ship remained the same, the starry but otherwise unremarkable background representing the dark (and convenient to render) void of space was still there, and the sprites for the alien ships had remained in the same shape.
If there was any improvement to be seen, it was that Namco appeared to have used the NES's technical capability to rotate the alien ships' sprites as they pass by, making them look as if they are taunting the player's ship (which would be thematically understandable, considering how clumsy and constrained the player's ship is compared to their mobility). There are also more colours and shading to the sprites compared to those in the arcade version, but these could have been difficult to appreciate.
The sound effects had been ported over quite whole-sale too; there were the usual droning, beeping and warbling that one would expect from Galaxian's arcade version. Unfortunately, there appears to be a glitch that occurs when alien ships get past the player's ship.
In Space Invaders, that would have resulted in a game-over. In the NES version, the punishment comes in the form of an annoying droning noise that plays as long as the ships that got by the player still live, and live they will, as they simply reappear at the opposing edge of the screen, i.e. the top edge.
All in all, the NES version of Galaxian would have been more fun to a player that had not played the arcade version, but for an older player that had gone through the arcade era, it would not have much value.