You should try to stay on the defencive and build up your turrets and get the most 
powerful turrets in your current age. The best turrets are:

Stone Age   - Egg Automatic.
Castle Age  - Fire Catupult.
Pilgrim Age - Small Cannons Then Upgrade to Explosive Cannons.
Mondern Age - Double Cannons, forget the missle launcher.
Futuristic  - Either get the Blue Ion Cannons, or just save up for 2 super soldiers.