Андрей
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I just read the thread with the poll, where everything is described. IMHO, if it is to be introduced, it should be as a separate Castle without a level. It would be too dangerous an overpowered unit; you could lose your main hero, for both sides. And then either you have to re-level them, which is not always possible because all the chests and monsters might have been cleared, or you have to end the game because it is impossible to continue.
The abilities of the units are scary even just at a glance; there are three entanglements + immobilizations, poison + biting – that's the end for weaker units.
Fire doesn't work underwater; that's the end for many mages, and water works on land; that's an imbalance. By the way, Meteor Shower is a completely earth-based spell.
There should be no difference in levels between the heroes for descending and ascending; otherwise, it will be too easy. Hit and run.
These are just some rough ideas that might help you refine something.
P.S. Although I voted against it, such a scale cannot but inspire respect.
P.P.S. GOOD LUCK!!!!!!!!!!!!!!!!!!!!!!!!!!!