Population
The population of cities is not an abstract number, but a concrete number. Small cities have a population of somewhere between 5000-10000 and large cities will probably end up with a population between 50000 and 100,000. Unlike many 4x games there are no dimining returns on population growth, it takes the same resources to grow your city from 10k to 15k or from 60k to 65k. You will have cities of all kinds of sizes in your empire depending on their importance and access to resources.
Population categories
The population of a city is distributed into different categories: workers, militia, guards and unrest. Depending on the unrest in the city some of the population will notwork at all. Many buildings require guards as upkeep. Militia is population that does not serve as guards currently but is available to serve as guards in the future in new buildings. The rest of the population is available as workers who can work in professions to produce resources. Both militia and guards increase the city defense which helps when the city is sieged by attackers.
Population Growth
Each race requires specific resources to nurture and grow their population. For most races this is food,but races like undead or golems will require other resources. Any additional food that is left over after consumption translates into population growth. However the population will only grow if there is available Housing. Each city starts with some basic housing,additional housing can be gained by constructing city districts.
Professions
Most of your population will be workers and can be assigned to different professions. Each race has access to a unique set of professions that reflect their culture. Professions create or transform resources. Farmers create food whereas alchemists transform mana to gold. You can only employ as many workers in a profession as you have profession slosts. These slots are gained from terrain, technologies, buildings and influential organizations like a merchant guild. For food for example there are 4 different professions: farmers, hunters, fishers and animists. Farming is the most efficient one but takes the most investment. You need to set up farms first which require fairly fertile terrain. Fishing requires lakes or oceans. Hunting can be done pretty much everywhere but certain terrain like forests is more suitable for it. And animists turn mana into food, so it is a terrain independent way to gain food but you need a lot of mana to spare for it.
Resources
There are local resources and empire resources. Local resources can only be used in the city they were produced, but you can transfer them to neighbouring cities by creating traderoutes. Empire resources are added to an empire stockpile and you can easily use them wherever you need them. What really spices up things is that what decides if a resource has an empire stockpile is not the nature of the resource, but the culture of your empire. Metal for example is for most races a local resource,but for dwarves it is an empire resource. If you play a race that is magically not talented then mana is a local resource, but if you decide to pick up the wizard leader talent then mana becomes an empire resource.
Traderoutes
You set up traderoutes manually by importing or exporting resources from a city to a neighbouring city. You set up how much of a resource per turn you want to export or import in a city. Not all resources can be traded this way, for example you cannot set up traderoutes for mana. Each city also has a cap for how many resources that city can export every turn and you can increase that cap with some buildings. Now traderoutes do not have a 100% efficiency, if you export 5 food you only get 4 food in the target city of the traderoute. The traderoute will create as compensation a special resource called tradegoods in the city where it starts and in the city where it ends. The idea is that when your traders embark on their traderoute they will trade a small part someof their goods in every village they come across for local goods and services. The tradegoods you gain from traderoutes are particularly important for merchant empires. For example the merchant professiondoes not create gold from thin air, it transforms tradegoods into gold.
In the trade network screen you can see the imports and exports of every city.
Here Harmsmar exports food to Donlin and also exports metal to Valesward (the metal is bought on the local market in Harmsmar).