Poseidon is an expansion game for Zeus: Master of Olympus that allows players to try their hand at building and running the legendary city of Atlantis. Poseidon adds six new campaigns to the game - a total of 45 new scenarios to challenge Zeus veterans. New campaigns aren't all that Poseidon adds to Zeus, there are a host of new structures that players can add to their cities.
The most obvious change is in the way you'll entertain your citizens. The citizens of Atlantis are the antithesis of Americans - they prefer science and learning to entertainment. Instead of providing theaters and the like to keep your citizens happy, you'll need to build libraries, laboratories, museums, and observatories.
The advanced knowledge of the Atlantis citizenry has allowed them to develop the skills necessary for pyramid building. Some of the pyramids available include shrines, altars, museums, and observatories - all of which will improve the aesthetics of your city.
Atlanteans are not all work and no play, though - they do enjoy a good horse race. To feed their need for races, you'll now be able to build hippodromes. These hippodromes can be as large and as elaborate as you like, because you can lay out the track yourself. You can even wind the track throughout your city, placing bridges over the track so that your citizens can still get around town. Build an elaborate enough track, and you'll attract out-of-towners (and their gold) to your races.
Finally, the lands surrounding Atlantis provide some new resources: deer, oranges, and cattle for food, and orichalic ore and black marble for industry and monuments.