those rare upgrades could be enabled if some resource is conected to the city. eg. having connected fire shard to the fort you could have a powerful firewall, a wildling camp in conclave could produce wildling shamans
End of Sentinemodo's quote
Good ideas! Wildling shamans are scary in groups!
Water shard creates large areas of stinking mud on the battlefield
A knights of Asok attached to the city creates a sally port that sends a unit of cavalry to defend the city. (trolls and ogres can do the same thing)
Life shard gives all units +2 tactical regen for city sieges.
Air shard gives Guardian wind, 2 shards adds an Air Elemental.