There's a tendency for variants to always be more complex than their originals, so this is my effort to redress the balance.
Every city is assumed by default to have an implicit garrison, belonging to the owner of the district.
The strength of the garrison is equal to the number of spires in the city.
Notional garrison factors are normally kept off map, and represented implicitly by the city symbol.
A notional factor belonging to the owner of the district is placed (becomes explicit) if:
Notional factors on the map are removed (become implicit) whenever any of the following occurs:
Notional factors can be killed in the usual way during siege combat or by foraging attrition. Notional factors that surrender are eliminated. Note that eliminated notional factors become implicit, and will return if the city becomes unoccupied.
Notional garrisons can do all the usual jobs like fire guns.
In addition to the notional garrisons, every power should be issued with a few large garrison counters. These are basically corps, containing just infantry. They may be slower than infantry corps, and hence recognisable.