If you are interested in Poseidon, then you probably already own Zeus. If so, then the question on your mind is probably whether or not you should buy Poseidon. The answer is: it depends. If you enjoyed Zeus, you'll like the expansion. 45 new scenarios go a long way towards extending your enjoyment of the game. If that's not enough, there is an adventure editor that will allow you to create more.
A few caveats should be mentioned, though. First, the coolest new structure is definitely the hippodrome. However, track construction can be tricky. If you are not careful, then you can find that you can't complete your track's circuit - a frustrating loss of time (and funds!). Also, if the combat aspect of Zeus was your favorite side of the game, you might be slightly disappointed in Zeus. While military units still exist and you may have to defend your city from time to time, the gods in Poseidon prevent you from attacking other cities.
If you're on the other side of the fence, the side that did not enjoy Zeus all that much, then you will probably want to pass on Poseidon. It does not change the game significantly enough that it is likely to sway your opinion.
In The End, This Game Hath Been Rated: 88%. If you enjoyed Zeus, you'll be quite happy with Poseidon.
System Requirements: 166 MHz Pentium CPU; 32 MB RAM; 2 MB Video RAM; 4x CD-ROM; 450 MB Hard Drive Space; Mouse.