Levy Units from Client States/Satraps

First, we need to play as a faction that can take Client States. The Greek/Successor, Roman, and Carthaginian factions (and probably others) can take Clients, Eastern factions generally get the option to take Satrapies, barbarian factions generally don't get this option. Sparta, as an example, has a somewhat limited roster and can benefit from getting some foreign missile and cavalry units from their neighbors. Our client state/satrap will also provide military support with their armies and navies which can help defend your borders.

There are diplomatic ways to take a client state/satrap, but most often these could be expensive or just plain difficult to achieve. I can use a purely military approach and use the Subjugate option after capturing their last city. My preferred method to take a client state/satrap is a mix of by force and then use diplomacy to avoid bloodshed. If I can manage to kill their armies in the field and then surround an enemy's last city (or multiple cities) with an overwhelming force, choose the maintain the siege option and then go to Diplomacy tab. With their last city surrounded and little chance of survival, the faction is much more likely to accept being my client while negotiating a peace treaty while under attack.

Now that I have my client state/satrap, I am able to recruit levy units from their roster. What I need to do is move one of my armies into their territory with empty slots for recruitment. I will now see a new icon to recruit levy units (replaces mercenaries). These come from the client/satrap faction's available units, allowing me to expand my roster to their units. In the example screenshot, you can see I am able to recruit Rorarii as this is the only unit Rome is able to recruit. Their recruitment cost and upkeep are the same as if they were my faction's units, which is much better than mercenary costs and with the same immediate recruitment like mercs!

So, to plan for how to use this to expand my roster I need to be a bit intentional on which factions to take as a client/satrap and how to let them expand. I don't want to take every faction as a client state because I won't own that territory for my own economy/recruitment/trade resources etc. Choose a faction whose roster might fill the gaps in what you don't have, so for Sparta I might want some extra cavalry options and non-spear infantry choices like Celtic swordsmen or axemen to give my army more flexibility where it lacks on its own.

I can anticipate what levy troops will be available based on the client/satrap faction's roster, and whatever they can recruit in the province my army is recruiting from. So basically whatever units they can get in that province, my army can get as levies from any area in that province. With that in mind, it's a good idea to let your client control an entire province to give them the best chance to build up enough economy and recruitment buildings to really make this effective. The levy troops I can recruit is chosen (seemingly) at random from the units available, and up to the number of regions owned by the client/satrap in the province. So if my client/satrap has 3 regions in a province, it should give me 3 units (or 3 of the same) when I move into their territory anywhere in that province.

So if my client/satrap is Rome and they have the building to get Hastati, Principes, and Equites, I will be able to levy those units.