This is not a story. This is, most likely, Math. If you're interested in the nitty-gritty details of building cities, then by all means, keep reading! If, however, you are only here to read stories and look and cool maps, you might want to give this a skip. Don't say I didn't warn you!
The Adventurer, Conqueror, King System has very straight-forward rules about empires, kingdoms, all the way down to itty-bitty villages. That said, it can get quite complicated quite quickly; I'm not entirely sure how someone could create an entire world with fully-populated urban areas without that being their full-time job. That said, I took some shortcuts. Short-ish, anyway.
The first step to populating a world is creating a map; with that in hand, I moved to the next step: populations. Cities marked on the map are larger cities, between Class I (20,000+) and Class V (250-624 families), usually (though there are a few exceptions). Easy enough; just slap in a few class sizes and names, and you're done. There are a few rules as to how many cities are allowed; based on the size of my country, I estimated the population to be around 1.1 million families. That's enough to have a single Class I city: the capital, Slowhaven. Norstun, Riverbend, Laketown, Sacred Grove, Wallace, and Wrapport make up the Class II cities; Emerald, Claywood's Town, Market Lake, Grove City, Soya, Plainsburg, Crossroads, Wallton, Crabbale, and Souton are all Class III; Evergreen Valley, Three Oaks, Ferric, Neptis, Ranchton, Wheatburg, Elmburg, Annhoradwy, and Wallend are Class IV; the two cities of Overriver and Herefed are Class V; and the tiny towns of Rivermouth and Idylton are Class VI. There are smaller cities, of course; tiny hamlets are present in almost every six-mile hex, though they are more community than city, and as such, have no market to speak of, or name for that matter. Bigger cities have bigger markets, more spells, and a better chance of buying expensive stuff, as well as exerting more trade influence on smaller cities. I'm still not entirely sure how I'm going to use trade, but as long as I had city classes, I might as well use it, right? Sure!
Knowing this was going to be a lot of math that I didn't want to deal with, I created a spreadsheet. I added the name of every city in each column, and the various trade goods in each row. With rather a lot of spreadsheet magic (this particular spreadsheet had 10 pages), I eventually ended up with the trade values for each city. First, I randomly generated the trade values for each city. Then, I added the various modifiers - bonuses for being near a lake or sea, forest or tundra, plains or mountains. That was the easy part. The next page was dedicated to distances - how far a city was from every other city, by land and by water. It was easily the most time-grinding section. Once I had that, I could calculate how a city affected any other city. Two equal cities move demand modifiers towards the other, while a greater and lesser city only moves the demand modifiers of the lesser city. For instance, Slowhaven affects Wrapport and Laketown, and Laketown and Wrapport affect each other, but neither affect Slowhaven. Even if a city is sitting on a diamond mine, if a much larger city desperately wants diamonds, the smaller city will want diamonds too.
A few interesting things stood out to me. First, as defined in ACKS, water is a very powerful tool. Cities along rivers are much better off, trade-wise, than cities along a road, because water "reaches" four times farther, as far as trade is concerned. That means Wrapport, at the southern end of the country, and Norston, at the northern-most tip, affect each other, as well as every other city on the lakes, rivers, or sea. It's pretty impressive. Other cities fared much worse; two cities, for instance, have no roads or waterways nearby, and are thus cut off.
Apart from Mareten, there are few civilized lands; the Fae really only have two cities, and they aren't close enough to trade (Fae cities are notoriously bad at interacting with each other... and building cities at all), the city-states of the Southern Desert are quite far apart, and most of the cities of the Wild Plains are too far apart to trade. The cities of Mejasta are close enough to trade, and then only between themselves. The other cities really could only have traded if there were a river nearby.
With those cities set up, I now have every trade route mapped, and the demands of each city set. In case someone wants to play a Venturer, I guess? I need to keep reading!
However, it has helped me understand my countries a little better, and develop their backstories. Two two tiniest towns in Mareten are both in wonderful trade locations; however, the people of Rivermouth are uncouth, dirty, and most likely racist people that nobody likes, and while the people of Idylton are nice enough, they've built their city on the flood plain near the river, and are constantly being flooded out, which means the city itself is not terribly old. Roads and highways are very important; the main roads of Mareten are wide, well-built and well-maintained roads, set with smooth stones that have had thousands of years to settle. The smaller roads have been worn into the earth, hard-packed by the feet of millions of people over the years. However, even after being settled for thousands of years, Mareten is still a fairly wild place. The wide open plains have few roads, and the northern mountains and southern forest are still very much wild places. The people of Mareten take this whole "free people" thing very seriously, and seem to prefer to travel through open plains than on a road. It's their feet, I guess.
Last but not least, all of this has made me really appreciate the scale of these cities. Assuming a family size of about 5 people, a city with 20,000 families is going to have over 100,000 people in it. For a medieval city, that's pretty huge - and yet, for a 2,000+ year old city, that's not all that big. These cities have sewage systems, plumbing, and firm foundations, built and improved (for the most part) over the years. The cities will have a richness about them - even if you can only add a single gold piece of improvement to a building every year, if it's around for 2,000 years, it's going to look amazing! With few enemies, the cities can devote a lot of time to improving themselves, instead of rebuilding.
That last bit is key. Putting these cities together isn't about math and spreadsheets; while complex calculations are a part of the process, they are only a foundation. If and when you make a country like this, be it for an RPG, a book, or just for fun, remember that. Names, dates, populations, trade routes, and all that math gives you a framework to build the real cities on: people, places, sights, and smells. Take the boring "Demand: Wood, +2." and turn it into words - Crabbale, though it has copious amounts of wood, still supplies most of Mareten; even with the woods at its back, Crabbale can never keep up with demand."
Then, take those words, and make them real. Crabbale, a logging city, has hundreds of loggers in the woods, chopping and cutting. The forest rings with axes, and trails radiate into the trees, paths cut into the woods to facilitate moving the massive amounts of lumber. If you're a lumberjack, or want to try your hand, there's always someone hiring. Of course, there are quite a few men who would like to try their hand at something else; if you're looking for a henchman who can swing an ax, then Crabbale is the place to look!
See? Starting with the boring foundation, we've added an air of realism to this thriving city. Add a few characters to hang around the taverns - folks anxious to get away from a life of forestry - and the city will feel as real as anything. From boring, to amazing, just like that.