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So, after playing more extensively, I'd like to share some more detailed thoughts.
Firstly, I did find a bug. The Elite Sorceresses (maybe regular ones too, but I didn't notice) cast spells completely unpredictably. They might only cast the spell on the target unit, or on all enemy units of that type, or even on all units. Is this intentional?
Also, I think the Castle needs to be strengthened; it feels quite weak compared to the standard ones. There are many requirements for building, which means that even building birds in the first week requires effort, and I won't even mention Nixies and Sorceresses. I suggest removing the requirement for a Forge for the Elite Pirates (you could keep it for the regular ones, I think), and also removing the requirement for Pirates for the Birds (keep the requirement for Sailors).
The first tier unit deserves special attention. In its current state, it's completely unviable, as it dies with a single blow. This is unacceptable for a Castle whose main strength lies in defense. The situation could be saved by granting it "Retaliation" (and even better, give it to the regular units right away, as there's really no desire to upgrade these creatures), or by giving it "Cleave." But the first option would be better. It could even be in a weakened form (like "Retaliation" only for male units), if that's possible.
The upgraded Birds deal good damage, but they also die very quickly, to my surprise, even under a hero with a special ability that buffs them. It's good if the opponent only has one unit: then you can save them for the final battle and enjoy the results. But what if there are several major battles ahead? In principle, it could be left as is, but I would still reduce their damage in favor of HP and defense.
The Sorceresses are not impressive. They don't deal much damage, and their spell has a rather insignificant effect. I suggest expanding the number of spells they can cast, so that there are truly useful ones among them (some kind of curse, forgetfulness, or slow). It would be great if they could choose the spells not randomly, but based on the situation (like the Djinns in Heroes 5). Also, something needs to be done about their upgrade: they hardly change at all when they become Elite.
The same goes for the Pirates. If the Corsairs still have some additional bonuses, then the Sea Wolves cost significantly more, but they don't offer much more in return. And in general, it should be noted that the Castle is very expensive, more than is justified by the quality of its units.
The Nixies, on the other hand, are simply excellent. They're not great at the beginning of the game, but at the end, they're amazing against high-level units. The idea is brilliant.
The Snakes are good, but they could be better. Although, I might not think so if I knew the meaning of the "Vengeance" ability. And although I didn't notice any special abilities, the battle showed that they are good tanks, probably due to their stats.
So, that's how it is. Just playing against the AI on one of the maps (with a lot of water, by the way) showed that playing as them is quite difficult. Even at the end, there are problems: when capturing cities, for example, they suffer more than anyone else, the Birds are killed with particular cruelty, and there's nothing else to storm with. Although Teleport and Phantom somewhat ease the task.
By the way, regarding spells: maybe I was just unlucky, but it's very difficult to get Earth Magic. Of course, water is water, but it's unpleasant to chase all sorts of fools around the map without a portal to the city. ;)
So, that's it, please take it into account.