How does the leveling system for culture cities events work?

In the weekly culture city you can fill crates with specific culture city resources (for example silk in China) to upgrade a specific Wonder in your culture city (for example the Chinese Wall in China). Once all crate(s) for the current level are filled, you can upgrade the current level of the culture city wonder. Once completed the bonus of the wonder will improve and you can claim the reward for unlocking this level.

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