A unit in Castle Overlord will gain experience points by participating in battles. After reaching a certain amount of experience points, the player can upgrade that unit by spending gold and materials. Stronger units will help the player win harder battles, and will also be more desirable in the NFT marketplace.
Upgrading units is not mandatory, but owning strong units is highly recommended in order to progress in the game. If they wish to, players may choose to recruit more units instead. Winning by quality or quantity, the choice is in the players’ hands.