As you go through the tutorial you'll notice that one of the NPC's will mention that you need a balance between managing your city and building your army. By now then your city is well on it's way to becoming an empire as long as you always have something being built. But how do you manage the balance between building an army and building your city?
Well first let's take a look at your resources and how to manage them and what purpose they serve
Gold - Used for researching technologies, buildings, training units, trading and increasing happiness.
Food - Used solely for your army. Used to train army units and increase army happiness.
Wood - Used for building structures and defenses.
Stone - Used for building structures and defenses.
Each of these resources are generated by a resource building for each resource they generate respectively. Your Goldmine generates gold, Stone Quarry generates stone and so on. Each one requires slaves in order to generate any resource and as the more slaves you have in them the more resources they generate.
Efficiently being able to build your city and train soldiers relies on how you balance your slave distribution.
I find that about a third in food, a third in gold, a sixth in stone and a sixth in wood works well for me. The reason for this is because I personally like to create more troops than upgrade my buildings.
This way you can farm more of the inactive players and haul in a nice amount of resources more quickly.
Basically in order to be powerful in this game is being able to build your army as fast as possible. Meaning you need to be able to find that golden ratio.
Remember that unused resources are resources that could be used to generate more money. At any given time if the resource generators are up to their highest level possible and you have resources to upgrade them you're wasting precious money. Try to always have a resource or cottage upgrading at least. And try to have all your slaves working at all times. You can always move them around.