A level ten player facing a level twelve player starts the match at a massive mathematical disadvantage before a single card is even played.

Many players inadvertently stall their base progression by hoarding resources or ignoring the social systems of the game.
The Primary Source: Upgrading Cards
The vast majority of your account experience will come from the simple act of upgrading your cards.

Upgrading a card from level one to level two costs virtually nothing and provides a very high amount of XP per gold coin spent.
Your deck is your sword.The cost is microscopic, but the XP bump is significant.Do not upgrade useless cards to high levels just for XP. The Secondary Source: Clan Donations
The second most consistent way to generate base experience is by actively donating cards to your clanmates.

This is why joining an active clan of fifty people who constantly request cards is absolutely mandatory for free-to-play users.
Guild RequestXP RewardSkeletons, GoblinsSmall amount of XP per card, but you can donate hundreds of them dailyRare and Epic CardsLarge burst of XP, but highly limited in how many you can give The Impact of King Levels
In close matches, the winner is often decided by a tower surviving with less than fifty hitpoints.

Never neglect your base progression; it is the silent protector that wins games.

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