This game is the best in series and is one of the best RTS games to date, and here is why.
In my opinion, the major reasons why this games deserves so much credit are: the atmosphere, addictiveness, graphics (2D) and perfect amount of in-game features including units, buildings and stuff.
To me, the sight of all trees cut off, out of their stumps, and farms set all around the map, is so fulfilling and therefore this game urges me intensively to play it again and again.
At first, I ought to explain you that this game does not differ too much to its predecessor except the new civilisations added in and some new units there. Pathfinding is improved, but still not at the level that we all expected to.
The game starts in the Stone Age (you can set any age you want to), the age whereby you are not able to use any of the researches and upgrades. Then goes the Tool Age, still inferior but provides some good stuff out there. Once the Tool Age comes, we proceed into Bronze Age, the age when the game begins to take some serious economical, technical and religious elements. And finally comes the Iron Age when buildings start to look really magnificent and all features are open to use.
The thing that brings about the atmosphere to be so credible is the soundtrack which gives us feeling like everything is "at that time", and personally I like it so much. I would say that in-game sound effects are pretty much decent.
In spite of the fact that Age of Empire I has been released in 1997 (expansion one year later), I still enjoy the artworks and graphics at all and deem them rather beautiful. All the civilizations have their unique look (5 different architectural styles) For that reason, each of them has its own look and charm.
Four resources are there to be gathered: food, wood, stone and gold. Each of them has its main purpose. Food is supposed for living units and proceeding into new ages, wood for constructions/ships and some units, stone mainly for walls/towers and gold for the Iron Age, strong units, priests and some technical/religious researches. Its importance grows as the game goes on.
As I said, gameplay is highly addictive. Since the quality of gameplay is not easy to describe, I recommend you all to try this game because you are not going to regret, unless you cannot digest all its "2D awfulness" (although I like it).
The only flaws about this game are its still frustrating pathfindings (despite it is corrected a lot) and some interface lacks. Otherwise, this game seems to be unbeatable, even today. I think it is better than Age of Empires II where buildings are too large and does not leave too much room for maneuvering. To me, this is the best Real Time Strategy game to date.