Garrisoning seems reasonable to me on the one hand, but I'd incline to
see this as a short term thing only--a garrison in a castle will waste
the income if no one is there at month's end to check the books (they pay
their salaries, and the rest is misfiled...). A garrison will ONLY allow
it's "owner" to enter (inconveniencing anyone trying to enter), etc.
TOwers and so forth that are garrisoned during the turn need to be
reoccupied by turn's end, or the troops, unpaid, desert, and the location
goes empty.
The garrison is supposed to be a convenience, not a way to build a series
of locations all held by a single noble, who makes the rounds collecting
funds....