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I mentioned that the idea of such a castle also occurred to me. As the main inhabitants of this castle, I envisioned lake elves. The reason is simple: they are mentioned in the Chronicles of Ashan, and it is at least known that this people helped the rebel elves of Tuidhana at one time. So, there is no need to explain their existence or weave them into the story; it has already been done. All that remains are additional, more subtle details.
Someone might say that there are also mentions of sea elves (a cultist bears their name). However, they are not present in the history; perhaps it is just an Easter egg, like the tank.
Another option: nagas. Unfortunately, there is something wrong with them. They actively participate in the events of "Heroes 6," but in the fifth game, there is only an uncolored model of them (although the model even has an animation). If they were to be made the main people, the question arises: where were they before this time, when such global events were happening throughout Ashan? Similar questions already arose regarding dwarves and orcs, but in the case of the former, the developers justified it by saying that dwarves are very peaceful, and orcs were obviously enslaved and did not have the physical ability to make themselves known. In the case of nagas, we would have to come up with some more excuses... That's not good. However, I would be glad to see them as a secondary people in some kind of aquatic faction.
There is also a lizard model in the game; in principle, that could also be done.
Although, these are not the main issues right now. The main thing is to implement the software capability to introduce at least some kind of castle.