In my opinion, Neptune should be made 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) around like on land, but when it goes ashore, it moves slower. To do this, place volcanoes on the water and extract sulfur from them...
Also, it is 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!
Added after 42 seconds
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 it cannot be placed in places where troops disembark from ships). It can be divided into 2 types: strong and long-lasting, and weak ones.
DeniusM:cool: