Maybe it's already been decided here :) But I'll add my two cents:
In my opinion, Neptune should be a 7th-level creature (after all, in mythology, he is the king of the seas), and the spider doesn't really fit (it's not a water strider), maybe it would be better to use a bloodthirsty shark or a giant whale.
If the Castle is not placed on a separate level, but on the water, on the standard 2 levels? Let it move (swim) like on land, but when it goes ashore, it moves slower. To do this, place volcanoes on the water and extract sulfur from them...
It is also not desirable to disrupt the balance between good and evil, i.e., in response, come up with another faction that has the same rights on the sea. And creating an entire level for one faction is too much!
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And I'll suggest another useful feature:
create a "Bermuda Triangle" on the water. Other factions cannot see it, but when they enter it, they lose part of their army. It would also be useful to have restrictions on the size of the area, the duration of the effect (several turns), and the distance from the shore (so that they are not placed in places where troops disembark from ships). It can be divided into 2 types: strong and weak, long-lasting.
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In general, here's the thing. For those who have read (those who haven't read, read it first) the topic for the poll "What is your attitude towards the possible introduction of a third level of the map and a faction?".
An idea has appeared, and it needs to be discussed specifically in this topic. Express all your opinions here, but not in the topic mentioned above!
So. Since we have planned that in some places the underground sea is connected to the surface sea, and there is a "Sphere with Water and Air" for entering the water, there is no need for ships!! Indeed, isn't it silly to sail on a ship if the hero has an artifact that allows him to move underwater?! In this case, the "Navigation" skill also disappears and transforms into this "Sphere...". Instead of the "Navigation" skill, our "Water Gate" skill is introduced.
We can explain when and why ships disappeared in the history of the faction, which (and this is natural) is connected with the history of the world of Ashan.
Ships can be kept simply by making the underground sea and the surface sea not connected. But... I would like these seas to have places of connection.
Express your opinions here.