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Here, I also want to share my thoughts on the three points mentioned above:
First: I've always been indifferent to which Castle I'm facing (except for Life vs. Life), because all Castles have their strengths and weaknesses, and it all depends on how developed your hero is and what kind of army they have. If your hero is strong, you shouldn't be afraid of any opponent.
Second: A worthy opponent is one who is equal in strength to you, but this doesn't mean that all other opponents are unworthy; it's just how I define it. All the others are simply good, strong, weak, etc.
Third: Not long ago, I was only playing against the AI, and they were starting to get boring because even a strong AI opponent required playing on Champion level or sometimes on level 3 (I forgot the name).
Then I tried playing against a human, and it was a completely different game...
Playing against the AI is the first stage of the game; then, to maintain interest, you need to play against a human because only by playing against a human can you see the game in all its glory, come up with many non-standard solutions, and the AI is monotonous.
The only thing that bothers me is that the game takes a long time