The empire fort garrison are good all the way to tier 3. You can defeat massive hordes of enemies utilising the choke points and elevated areas of the map. However, on later tiers, the garrison become much more suited to be a standard army rather than defensive force. Stuff like Hellstorm Rocket Batteries or Great Cannons have no use since they have a hard time shooting onto enemies at a lower angle. Handguns are useful but not as good as crossbows who can arc their shots. The only real "upgrade" is the melee infantry becoming halberdiers and great swords.
So the garrison itself isn't too bad, but some of the most impactful pieces of it (siege and ranged) are completely useless and usually lead me to auto-resolving any battle once I decide to get the later tiers. My recommendations are as follows:
- keep the 2 mortars and a great canon as artillery (potentially update the great cannon to a hellblaster)
- keep the crossbowmen as the ranged units
- in later tiers, change the artillery to become the elector troop versions
- in later tiers, change the ranged units to become the elector troop versions
- keep the melee infantry as is
- keep the cavalry as is