In a previous post I talk about how the military of city-state is organized. For those who care about realism it looks implausible as rigid table of equipment and organization did not evolve until much later in history.
I could fall back on the excuse "I am the referee and I can do whatever hell I want". There is a bit of that in my setup for city-state but in general I like to keep things as realistic as possible. I find that keeping things historical with a touch of the fantastic makes for a more interesting game then either extreme.
The key for applying table of organization to fantasy society to remember that recruitment of armies varied over the centuries. If asked the vision that most people have in their head is the feudal lord issuing a call to his vassal to assemble at a place to march to war. For those who seen the Lord of the Rings movies this vision is much like the mustering of the Rohirrim at the beginning of the Return of the King.
However the feudal muster was in vogue only for a limited time centering around 1000 AD. The problem is that it wasn't reliable. In fact the only reason feudalism established itself was because there was nothing better. The collapse of the money economy after the fall of Rome left little choice for those in power to adopt a feudal system to support the troops they needed.
As the economic situation improved during the High Middle Ages the Kings of Europe instituted a better way of recruiting the troops they needed. First they started to commute the military service owed by their vassals into cash payments. In turn the Kings used the money to hire mercenaries.
The interesting part was how they hired mercenaries. There was no merc-mart or emporium where individuals and companies could be hired. The system they used was built on it's feudal predecessor.
When the King of the High Middle Ages wanted to go to war, he and council figured out the size and composition of the force they wanted for the companion. After doing this they divided it up into individual contracts. The King would turn around to trusted vassals and retainers and offer one or more contract. The contract stated that for a specified payment the holder will appear with a certain amount of troop equipped as stated in the contract.
Rarely the contract holder had enough men and resources to create the force the contract required. Instead he would create sub contracts and offer them to HIS trusted retainers and vassals. This process repeated itself down to the landed knight accepting a contract to bring himself, his squire, and three yeoman to the mustering point.
In general the contract holders got a free pass saying that their feudal obligations for military service were fulfilled by being part of these contracts. The rest of the hierarchy were paying the taxes and feudal dues to the king so he could pay the contract holders.
This web of contracts relied heavily on the social network created by the feudal society. Which makes the concept eminently useful for a roleplaying campaign. Having done the Baron a favor the PCs could find themselves being offered one these contracts. Not only they would get paid for fighting but also a share of any plunder gained on the campaign. In addition much of a medieval campaign is comprised of skirmishes rather than pitched battle so there is opportunity for individual glory.
In my Majestic Wilderlands campaign, the Overlords of City-State have fully embraced the contract system. It evolved because the clan based society of the Tharians made it difficult for the Overlord to recruit troops reliably. For the last four generations the Tharian Overlords have successfully converted many of the clan obligations of service into direct payments. In addition the Overlords have won substantial territory that they own personally outside of the Tharian clan system.
As a consequence the Overlord forces have been divided into two Legions both roughly 5,000 men. The first legion is a permanent standing army maintained by the Overlord personal revenues. While the second is comprised of the remaining Tharians who still fulfill their obligations with personal service rather than cash payments.
Both are organized and recruited by the use of mercenary contracts. The main difference is that the first legion contracts are renewed on an annual basis. Some units of the first legions have existed continuously for over 80 years. These unit often have an associated fraternity of former veterans that act mentor and patrons for the current muster of troops.
In contrast the contracts of the second legion are temporary rarely lasting longer than a season. They are generally offered to those who have personal service requirements to fulfill. The initial contract holder relies on the social network provided by his clan and its allies to fill out the muster. In general the forces raised for the second legion are used for garrison and patrol while the forces of the first legion are assembled to fight the Overlord's wars and conflicts.
Rarely the system works out perfectly and many contract holders of both legions come up short. At this point they turn to the Mercenary's Guild of City-State to hire one of the numerous free companies. Established two generation ago the Mercenary's Guild is supported by the Overlords as a useful mechanism to keep order among the free companies. A free company that is sanctioned by the guild is cut off from earning an honest living and must live the much more difficult life of brigandage, piracy or heaven forbid adventuring ;)
All of this came about because novels like the Black Company became popular in the 1980s. There was a sizable segment of player who wanted to play mercenaries. One of my best campaigns started out as a mercenary campaign where one player owned a ship another the leader of the mercenary company. They made a lot of gold raiding the Skandians for the Overlord.