Your first priority in this mission is to advance to the Classical Age. To do this, you must accumulate 400 Food and construct a Temple, which serves as the prerequisite building for advancement. Start by establishing a robust economy, aiming for 15-20 Citizens. Assign them to Food, Wood, and Gold to ensure a steady resource flow. Once you have the Temple completed and the necessary resources, click on your Town Center and select the 'Advance' button.
In the first mission of the "The New Atlantis" campaign, "Lost Liberty," you will play as the Atlanteans. Upon reaching the Classical Age, you must choose a Minor God. Prometheus (for Prometheans and the Valor God Power) or Oceanus (for Caladria and the Carnivora) are strong choices. Prometheus is generally preferred for aggressive early-game play, as his Prometheans are excellent frontline Myth Units. While the age transition is in progress, continue to train Citizens and research economic upgrades like 'Hand Axe' and 'Pickaxe'. Keep your Oracles active to expand your line of sight and scout for incoming enemy raids.