One thing that Spartan Wars has that I really like is the integration of Gods into the game. Gods serve a couple of functions. They can boost your city's economy by being worshiped or they can lead your army into a battle and give your troops a buff. Each god has it's own abilities and they get better as you level them up. Not all gods are equal though as you will see. Poseidon for example at level 4 gives a 40% buff to food production when worshiped.
CAREFUL because some gods offers a buff for PROTECTION that is 30%. Don't mistake that for PRODUCTION which will be 10%
You can also sacrifice your gods to earn faith. Faith is basically experience points for gods which you distribute in order to level them up. But for some reason you can't go over level 4? Why would 4 be the cap? I have Zeus and I'm leveling him up but I still can't level up my other gods!
The reason for that is because you need to join an Alliance and worship Zeus in the Statue of Zeus.
Your alliance all comes together to worship Zeus and when your alliance's Zeus has enough experience you will be able to further upgrade your own gods.
Be careful about sacrificing though. I would recommend keeping the gods you get until you're able to take on the Mystic Cave level 5. Since gods don't yield that much faith anyway. You only need to have one for each march that you want to do simultaneously plus the amount of cities you have. I like to have about 3 gods to send on marches and 2 for my cities. Some people have 7 for marches and 2 for their cities. This may be the optimal amount if you are getting lots of faith.
Any more gods than that may just clutter your temple with useless stand-by cards.
Also if you attack people enough you may also stumble upon a god card as a reward. God cards are rewarded at random but it seems like attacking higher power cities will give more cards.