Fortifications
Better fortifications and higher walls around the keep, also increases the max garrison limit since it provides more space for the resident troops.
Levels
- Level 1: +50 bonus to garrison capacity +30 increase to prison capacity
- Level 2 (1,400): +75 bonus to garrison capacity +45 increase to prison capacity
- Level 3 (2,800): +100 bonus to garrison capacity +60 increase to prison capacity