I , along with some other folks misunderstood the function of resources so I thought I’d share this info.
"How do keep improvements work? Are they visual (bigger walls, towers, etc.) or just an increase to the keep’s stats?
Each resource near a keep controls specific upgrades for it: farms upgrade guards, mines upgrade walls, and lumber mills upgrade the keep’s doors. Walls and doors gain more hit points from these upgrades, while guards get better abilities. There are some visual upgrades: walls receive improvements to their corners and sections near the postern doors. These upgrades occur over time as long as the same alliance owns the resources and the keep. "
You can Resurrect (edit : I was wrong about this) , but not transit to, a keep even if an enemy controls all of the resources. When the keep is flagged as under attack you will be unable to fast travel to the keep, regardless of the status of the resources.