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As practice has shown, phoenixes are indeed a dangerous unit. And most importantly, building a dwelling of level 7 in the elemental castle is much easier than in many other castles. If everything goes well, you'll have a couple of phoenixes by the third week, and then you can crush everyone. This cannot be said about Giants; building them costs very little, but you need a huge amount of resources to be able to buy a level 7 building, plus Giants are completely useless. Phoenixes are a dangerous unit, and that's a fact.
And I'm curious, who would prefer level 6 units? It's very interesting, and level 5 is also very interesting.