I use the mercs to garrison the newly-purchased castle / city until I can get some cheapo militia there to do the job - then I either disband them if I have no immediate use for them, or they join my armies which are going after the nearest rebel settlement (usually created by massassasination)

to be honest I'm making money hand over fist in this campaign, so upkeep is the least of my worries.

I thought this campaign would be hard to complete in the time limit, and at first I struggled because I was trying to do it all through religious conversion and spying to raise unrest and make cities revolt.

i realised that was only going to work for a limited number of settlements because the unrest from spies maxes out after five, and the cumulative penalties are only enough for settlements far from the AI faction's capital - plus capitals themselves can't revolt

but now I've got the assasination squad up and running, and the coffers are overflowing I think I can pretty much poison and buy my way to victory

btw I have 25 territories after 78 turns - so i have about 140 turns left to get the remaining 20

I'll update the main thread (https://forums.totalwar.org/vb/showthread.php?t=87178) when I make some more progress