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If we're talking about the classic SoD (for example, in platinum difficulty), then most players consider the Conflux to be the strongest castle.
There are very good reasons for this, in my opinion, four of them:
1. The phenomenal speed and usefulness of the upgraded first level units;
2. The strength and HP of the second-level units, combined with their ranged attack;
3. The phoenix production rate (remember, it's twice as high as the seventh-level unit production in other castles);
4. The ability to learn any magic school directly in the castle.
In addition, the castle has some very strong heroes.