Initially, it might not be advantageous, but gathering troops becomes more powerful over time. Discarding it in the first couple of weeks won't drastically reduce its effectiveness.
- To the nearest one at the time the ability is acquired
- To the most developed one in order
- To the one that came under the player's control earlier
- Provide a selection menu via script
Everyone uses chains; troops are transferred within a day.
Subjectively. But objectively, it's less versatile. I won't argue about its simplicity, though.
On the other hand, if you take a normal map (of any size), with adequate neutral creatures, where you can effectively break through with a large army, the picture is the opposite: city-based troop gathering is useless (you have to wait for the beginning of the week, hire a creature, wait for it to deliver reinforcements), while equipping new soldiers directly at the army's location is in demand.
It's worse when a creature (a hero without an army) takes the city-based troop gathering perk. With my implementation, it wouldn't be able to provide bonuses out of thin air, but with a link to the city, it could. So, you either need to make it an overpowered perk because of creatures, or a perk whose power should increase with level. In that case, it's a useless perk for the main hero at the beginning of the game.
And finally, the most important thing is that it's a level 1 perk, i.e., something that can be offered almost always when leadership is available, regardless of the player's desire. On Monday, do you want to be annoyed seeing a perk in the distribution that only increases the number of troops in the city after a week (and in the army after a few more days)? There might be a solution: give creatures to the city immediately upon taking the perk, plus weekly. But such a perk already exists for the Barbarian in The Shadow of Death: summoning a manticore, with the difference that the manticores will arrive once and weekly directly in the hero's army, which is much more practical and convenient.
Also, I want to remind you that there are also role-playing maps without cities at all (hire creatures from external dwellings or via scripts). What should the perk be tied to in that case? And yet, it's a basic perk, i.e., it will be annoying to see often, especially without it, access to a tasty guild leader is blocked (or should we support the perk as a purely passable one? That's not our choice).