Well, city garrisons don't protect people _outside_ the city,
at least not against attacks by savages (a special case?),
so they probably don't interfere with attacks they can't see
(such as attacks against a tower in a castle in a city).
Whether they defend you against an attacker who is attacking
from the city against your location, well... I would expect
them to, since the combat is going on in the city itself
unless and until the attackers win, but that's common sense
rather than what the game does. Try it and tell us what happens! :)
To get real static defenses, you'd need to garrison any kind of location,
from towers to cities to provinces, and they could all
coexist; however, the rules state that you can only have
one garrison per province, so if you are anywhere except
standing in the middle of a city (or a garrisoned province
without a city) you have to have all your own defenders
with you. Which means garrisons are pretty much useless
except for gathering taxes.
If anyone knows differently, please enlighten us.