The Blacksmith is vital for improving your melee and ranged units. Here you can research 'Sword Upgrades' (increases Footman/Knight attack), 'Shield Upgrades' (increases Footman/Knight armor), 'Arrow Upgrades' (increases Archer/Elven Archer attack), and 'Bow Upgrades' (increases Archer/Elven Archer range). Prioritize these based on your army composition. For example, if you're relying heavily on Footmen, get their attack and armor upgrades first.
