To progress through the game, you have to evolve to other ages. There are 5 ages: Savage Age, Dark Age, Feudal Age, Castle Age, Empire Age and Firearms Age. As you progress through the ages, you gain the ability to build more buildings and higher level troops.
In Savage Age, the 1st Age, you can build the Town Center, the Warehouse, the Tavern, the Research Academy, Barracks, and Civilian Houses. A special building is the hero monument, which costs 100k gold. If you don't build a lvl 1 hero monument in Savage Age, you need to buy a card to build it in a higher age. You can recruit T1 Peltasts. You have beginner's protection when in Savage Age.
In the Dark Age, the 2nd Age, you can build the Archery Range, Military Armory, City Defense Office, the City, and Walls/Gates/Towers. You can recruit T2 Peltasts, T2 Archers, and T2 Pikemen. You need 600 glory, and a level 10 hero to evolve to Dark Age.
In Feudal Age, the 3rd Age, you can build the Stables, Trade Post, and Civic Center. You can recruit T3 Peltasts, T3 Archers, T3 Pikemen, and T2 Cavalry. You need 2,500 glory, 200 Spirituality in your City, and a level 20 hero to evolve to Feudal Age.
In Castle Age, the 4th Age, you can build the Arsenal and Hero Academy. You can recruit T4 Peltasts, T4 Archers, T4 Pikemen, T3 Cavalry, T2 Giant Crossbowmen, and T2 Ballista. You need 10,000 glory, 1200 Spirituality in your City, a level 10 Merchant, and a level 40 hero. Empire Age people can plunder Castle Age people in the same Civ, and vice versa.
In Empire Age, the 5th Age, you can build the Castle. You can recruit T4 Peltasts, T4 Archers, T4 Pikemen, T4 Cavalry, T4 Giant Crossbowmen, T4 Ballista, and your country's special T5 units. You need 40,000 glory, 4800 spirituality in your City, a level 30 merchant, and a level 60 hero. Empire Age people can plunder Castle Age people in the same Civ, and vice versa.
In Firearms Age, the 6th Age, you can upgrade buildings past level 20 and recruit the new T6 units. There is a T6 unit of each type for all civs. You need 80,000 Glory, 6,000 Spirituality, a level 100 Merchant and a level 90 hero. In this age you can only plunder people who are also in this age.
How To Progress in Age
As shown in the picture below Click on the small flag witht he arrow in it. This will open the window underneath, on the bottom left of said picture shows the Requirements needed for your next age. If you meet all the Requirements then you can click the Evolve Button on the bottom.