In most arena battlers, your account level (often represented as a King Tower or Base Headquarters) dictates the health and damage of your main defensive structures.

Many players inadvertently stall their base progression by hoarding resources or ignoring the social systems of the game.
Leveling Up the Deck
However, the ratio of gold spent to experience gained changes drastically depending on the level of the upgrade.

If your sole goal is to level up your king tower quickly, upgrading all of your low-level cards evenly is the most cost-effective method.
Never compromise your primary win condition just for base XP.If you have excess gold, upgrade your level 1 cards to level 6.Balance is required. 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.
Donation TypeXP RewardBasic TroopsConsistent, reliable progressionRare and Epic CardsLarge burst of XP, but highly limited in how many you can give The Margin of Victory
If your king tower was one level higher, it would possess several hundred more hitpoints, turning that loss into a win.

A higher level tower might kill a goblin in two shots instead of three, preventing thousands of points of chip damage over a match.

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