Each god has an ability that should be used as often as possible to take advantage of their cooldown periods. For example, the starting god, Zeus, has Chain Lightning as his initial ability.
Defense
Your gods will be inaccessible during the defensive turn. You will have to rely on your troops. Academies and defensive towers will be required for their effectiveness. You can summon troops at any building desired every few seconds. Do this while your abilities are on cooldown to avoid being overrun.
Gems
Complete the achievements listed on the side bar. Each achievement has three tiers. The more tiers you complete for the same achievement, the more gems you will earn starting with 10, then 20, and ending at 30. Build a fountain, a decoration-type building that produces a gem every ten hours. Remember to collect them when they become available. Other gem producing buildings will become available as you progress. You will be awarded gems each time you level up. Consider fighting opponents under the hard difficulty setting. The stronger your opponent's base, the more experience you will be given after victory.
Gods
Consider choosing Athena as the next god to partner with Zeus. Her health is very high and she can take a lot of damage. Her Aegis ability will reduce incoming damage by 70%.
Upgrades
Return to the Pantheon to upgrade your gods by spending gold to improve their health, strength, or abilities. Upgrade your defensive towers before the various temples of the gods, . The gods' temples convey passive bonuses to your gods and troops. However they will require heavy upgrades before their effects become noticeable. Instead, start by focusing on your arrow towers, garrisons, and other defensive structures.