Nargott
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In the new concept, it's more of a division into 4 groups (like in Heroes 4):
Tier 1-2 (upgraded with Fortress, Stronghold, and Castle)
Tier 3-4 (upgraded with Stronghold and Castle)
Tier 5-6 (upgraded with Castle)
Tier 7 (not upgraded)
Tier 1-2 (upgraded with Fortress, Stronghold, and Castle)
Tier 3-4 (upgraded with Stronghold and Castle)
Tier 5-6 (upgraded with Castle)
Tier 7 (not upgraded)