From Rise of Kings Wiki
Overview Chronicles tech tree Bronze Dawn tech tree

In the Castle Age, towers will be upgraded automatically to a different structure: the Keep. Keeps have more hitpoints, more armour and better line of sight. You must build a Fort and research Fortifications there first in order to upgrade your Towers.

The researching of Divine Mandate by Orthodox or Asiatic factions during the Castle Age upgrades Keeps into Bastions. Bastions are substantially stronger, and fire more devastating projectiles on opponents. The Bastion also has a more powerful range.