Schizoid-man wrote: »

How close to a keep do you have to be for the Support Passive - Combat Medic to be effective? And does it work at both friendly and enemy keeps?

You have to be within the keeps range. I you pull open your menu at the bottom right it'll say your current location. Make sure that says keeps name. Not keeps grounds. Also if pay attention all castle will have a border around the keep on map sometimes pillars sometimes rocks ect. Usually that's border to show you how far keeps range is and where mage guards can atk you if enemy keep