The collectible troops, spells and buildings. Duplicate copies of a card plus Gold are spent to level that card up; owning and leveling cards drives most progression.
Duplicate copies plus Gold buy the next level, and every rarity now tops out at Level 16. Rarer cards need far fewer copies per level but those copies are far harder to find — which is why ladder decks lean Common and Rare long before collections max out.
At equal skill, levels decide close games — a level-14 Knight survives interactions a level-11 one doesn't. That's the honest reason under-leveled players favor spell-cycle and siege decks, where precise play converts better than raw stats.
