Catch
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Then, every week, 2 Key Dungeons - a total of $5000 + 2 silver (I think).
Immediately after capturing 2 Key Dungeons, then +1 each week. The cost is 7000 + 2 silver.
I also used Hydras, but there's nothing better than Djinns for this. A few minutes of figuring out the tactics, and there they are, capturing Key Dungeons in the first week :)
It's harder on the online map, but I remember how my 4 Liches and five Skeletons took down the dragon nest, losing all the Liches and 1 Skeleton.