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I captured the mines and fields around the city (where the strength is 5000).
Choose a hero and click on the coral reefs. In the pop-up menu, click on the sword. Then you will see how many naga are there. Arrange your troops correctly (cavalry against archers, infantry against cavalry, and archers against infantry) and calculate on a calculator how many troops you need :). I saved up for less than a week, but you should hurry.
I will still take the second reefs, if anything, I will drop the key to someone else.