After playing Galactic Civilizations II, the original game feels like a staggeringly large step back.
Galactic Civilizations doesn't have a campaign, but its expansion contains two campaigns which aren't really worth playing. Like its successor the standard game is where the real purpose lies. You can tweak a few options while setting up your galaxy like the galactic map size, and you can customize the Terran Alliance extensively (you can only play as the humans in this game). After all of that is out of the way, you can begin your intergalactic space opera.
Galactic Civilizations has five other races, and its expansion adds an extra two allowing you to have up to seven major players share the galaxy with you. Like in most 4X games, you can interact with the leaders of these races by trading, offering tribute, demanding tribute, forming alliances, etc. The diplomacy is a bit simplistic, but it functions adequately enough for a game of this type. I found it a bit too easy to sway the other leaders to my side, though.
Like its successor, you can conquer the galaxy in multiple ways. There's the good old fashioned genocidal rampage (Conquest), ascension through technology (Research), forcing your delightfully awful fashion sense on others (Cultural), and forming something resembling the Federation from those Star Trek TV shows (Diplomacy). The AI can also achieve victory through these methods, and thanks to Stardock's very human-like AI it's very possible that they will before you do.
Visually Galactic Civilizations is terrible even for its time. It looks more like a mid 90s strategy game then a strategy game from 2003. 3D strategy games weren't quite the norm yet, but undetailed and cartoon-like sprites were out of date even then. I appreciate old school as much as the next veteran gamer, but it was more of an annoyance in GalCiv. The audio isn't much better, the music is fairly forgettable and seems more like a MIDI score.
I don't know if Stardock was intentionally trying to make the original Galactic Civilizations old school with its 1995 graphics and audio, but in my opinion it didn't work out too well. This may be because I've been spoiled by its successor, though; regardless there's not really much of a point in playing this game as Galactic Civilizations II improves and expands upon every aspect. I recommend playing that instead if you haven't, and if you have there's not much of a point in checking out the original game unless you absolutely must play it for whatever reason.
+Massive Scale
+Superb AI
+Depth
-Mediocre Visuals/Audio for its time
-Mediocre campaigns in The Altarian Prophecy
-Playing Galactic Civilizations II spoiled my perspective