I see that no one believes all these castles will become a reality in HOMM5... and they're right, but people need to write their thoughts somewhere, don't they? Moreover, these castles (if well-developed) could compete with the existing ones... anyway, I also wanted to come up with some kind of castle..
Let's start with the fact that while playing HOMM5, I haven't encountered a race anywhere that worships Shalassa—the goddess of water and seas, as it were... Indeed... Elrath—humans, Urgash—demons, Sylana—Sylvan elves, Malassa—dark elves... the dragon mage for the Silver Cities, Arkat—dwarves, Asha—undead.... even Silata humans once worshipped. But there hasn't been a single faction in the history of the HOMM5 world that worshiped Shalassa..
Now everything in order:
1) State name: Great Atlanta
2) Faction name: Naga
3) History: Their history begins in the League of Silver Cities.
The Blood Moon War (year 330 S.D.)—not only orcs appeared in this war.... The elves were crushed because they did not expect a demon invasion from the sea.
Mages came to their aid, creating other terrible creatures from elf volunteers who could easily deal with this flotilla. Everything worked out extremely well for the Haven... In the end, almost all ships were sunk, and the remaining demon lords, who were supposed to attack from land, were driven away by orcs and humans..
The question arose here: what to do next with these creatures?
The solution found itself.... Instead of freedom, orcs became slaves on land, and amphibians in the sea.. The orcs soon grew tired of this treatment and decided to flee slavery, naturally taking the fish-people with them..
A new stage begins 467-504 S.D.—the Holy War against the orcs
The escape from slavery is difficult for the orcs.... they scatter into tribes in various regions; others go to the southern shores, closer to the sea, and found their own small state and people—Naga.. The mages of the Silver Cities soon learn about this and announce a campaign with the goal of re-enslavement.... A day before the mages appeared in the lands of the Nagas, Shalassa—the goddess of water—comes to their leader and offers a deal. She will save them from inevitable defeat, but in return, they will become her servants.... The leader had no choice, and when the mages approached the capital Atlanta, they witnessed a strange phenomenon..... before their eyes, a smooth crack formed around the city and it sank underwater in a few minutes.... The ruler of the mages took the credit for himself, saying that it was a spell and he alone destroyed the entire city. From that moment on, only orcs heard about the nagas.. Meanwhile, the empire's huge sacred fleet sailed to capture the orcs, but by chance, it fell into a whirlpool and was completely destroyed, although for believing humans, this became a sign that Elrath does not wish for the enslavement of orcs. The war ended, but the orcs saw the truth; near the whirlpool was a creature resembling a mermaid, and from that moment on, they began to call them water spirits.... No one ever heard about the nagas again... Many centuries will pass before the nagas return to land once more...

Now directly to the units:
1)Margol Slave—a small evil creature that throws whatever it can at enemies (shells, stones). They live in crowds and attack as a mob... Their power is not in strength but in numbers.
Abilities: shooter
1.1)Margol Shooter—these margols are more experienced shooters and bring some use. They try to hit the opponent in the head, thereby stunning them for a while, so that the enemy will think only of their headache rather than the battle.
Skills: shooter, powerful throw: there is a chance that the enemy will be pushed back on the ATB scale and temporarily will not counterattack.
1.2)Margol Cannibal—these were given weapons in their hands, if you can call it weapons.. However, a piece of old shell infects the enemy's wounds and thereby weakens them; moreover, these margols do not actually mind eating on the battlefield, which helps restore their strength.
2)Crayfish—as funny as it sounds, these crustaceans became the cavalry of the Nagas.... these creatures offered help to the nagas themselves when they first appeared at the bottom of the sea.... imagine the surprise of the snake-people that there are tasty crayfish 4 meters long and they can even talk.... :D
Skills: claw strike: the greater the momentum, the higher the probability that the crayfish will deal damage to a nearby enemy unit.
2.1)Crayfish Sorcerers—quite intelligent creatures who managed to master several spells and provide good support to other troops on the battlefield...
Skills: caster-ring of cold, ice bolt+claw strike
2.3)Hermit Crayfish—bloodthirsty on the battlefield, they can fall into a frenzy if one of the friendly units has perished... they also learned to inflict cutting wounds..
Skills: claw strike, bleeding) attacking an enemy there is a chance to inflict a cutting wound, which deals 25% of the dealt damage every turn.
3)Triton—somewhat resembles a sea dog. Attacks the enemy from a distance using spikes and simultaneously poisons them.
Skills: shooter, poison.
3.1)Triton Leader—at the head of each pack stands its own leader... it is the same here. If he is present on the battlefield, then the morale and combat fervor of other units also increase.
Skills: shooter, poison, leader's presence
3.2)Triton of Terror—the sight of this triton makes even friendly troops tremble... only the bravest dare to approach them
Skills: poison, frightening appearance: if an enemy unit stands or passes by it, there is a chance that it will turn around and move to the maximum distance, losing all initiative.
4)Sea Witch—a sorceress commanding storms, lightning, and the elements in general; with a regular attack, she summons a tornado that slows enemies.
Skills: shooter, caster-lightning, storm strike: reduces target's initiative.
4.1)Sea Siren—this is the so-called elite of sea witches, who have further perfected their mastery and acquired the ability to attack several enemies at once.
Skills: caster-lightning, chain lightning+attack also hits two nearby targets (like a chain lightning), damage to the next target is half that of the first, and to the third half that of the second (skill works if targets are standing close to each other)
4.2)Sirens of the Deep—these sirens fundamentally changed their concept of combat. Believing that there are enough shooters and mages in the faction, they focused on physical power and exhausting the opponent, which helps them well in battle
Skills: parasite: implants parasites into the bodies of an enemy unit, deals significant damage + upon death of this unit, a special squad of margols appears.
5)Giant Turtle—these formidable creatures were enslaved by the nagas and over time became an undeniable part of their nation.... huge turtles are used not only as a combat unit but also as a ram and transport.
Skills: transport in battle: can swallow one friendly small creature for two turns and carry it inside (the swallowed unit takes no damage)—for this skill, the turtle spends only half its initiative+can attack gates...
5.1)Maata—differ from ordinary ones only in that they have growths on their shells, which cause considerable pain to enemies threatening their lives.
Skills: same + 10% of dealt damage is returned to the enemy
5.2)Stone Turtle—these turtles are over 100 years old and their shell and skin are covered with stone growth, which gives them colossal protection and the ability to become invulnerable.
Skills: same + can hide in its shell for a while, thereby becoming an obstacle to enemies while remaining untouched itself.
6)Ray—these creatures also possess intelligence, but they were not deprived of strength either... When the Nagas first go onto land, they will learn all the beauty of these creatures.. Thanks to their body, they learned to fly, and the sharp sting has retained its lethality...
Skills: flying creature, large creature+sting strike: attack an enemy standing close with a sting, whereby at least 1 creature dies and the hostile unit loses part of its initiative.
6.1)Manta—this is slightly larger than an ordinary peaceful ray, but if someone offends its Naga allies, it will undoubtedly enter the fray... they say that its touch can destroy any spells..
Skills: flying creature, large creature, sting strike, upon striking there is a chance to dispel a negative spell.
6.2)Kaotal—this is half-lizard half-ray, also capable of flying. Unlike the manta, it is a very ferocious creature, although this is more of a plus than a minus... Due to their character, they can latch onto a victim for a certain amount of time..
Skills: large creature, flying creature, deadly grip: kaotal can latch onto a victim, covering the hostile unit with wings; in this case, the victim takes damage and does not counterattack. When the turn of the affected unit comes, it receives damage again; if it skips a turn, the kaotal will also attack it; if it moves, it is knocked back 1 cell (note: kaotal can be attacked while "sitting on a unit", but the unit he has hit will also take damage, only in a reduced amount)
7)Naga Fighter—when nagas first moved to the depths of the sea, Shalassa endowed some nagas with unseen strength. Subsequently, descendants of these nagas also possessed it and received special respect among the troops—they all possessed a strange fusion of immense strength and magic. Mostly these elite troops are armed with a heavy trident; only some of them use armor, as their scales protect well against various attacks... Rumors say that the goddess granted them immunity to sea magic and constant luck in battle.
Skills: large creature, immunity to cold and lightning, constant luck (not lower than +1)
7.1)Naga Guardsman—a Fighter with a capital F... there is no creature in the sea that can compete with a naga guardsman in battle. The longer the battle lasts, the more ferocious these warriors become. Eyewitnesses claim they saw with their own eyes how one such naga, fighting off a whole crowd of enemies, still won... I wonder how they themselves survived? Also, their trident is endowed with magical power, making them even more dangerous enemies.
Skills: same+attack and damage parameters increase significantly every turn+10% boulder strike
7.2)Naga Defender—this fighter is famous for his valiant behavior on the battlefield. He protects not only himself but also other allies; added armor, which although reduces their eagerness to attack, makes them practically invulnerable. Their trident is also endowed with a slightly different but magical power.
Skills: same + blocking: there is a chance to block a strike in close combat (if a friendly creature stands nearby and the enemy hits it, approaching the Naga Defender closely, the skill can also work)+ 10% lightning strike.

Hero—Lord of the Sea
Specialty and special skills: as you noticed, this race has many abilities that "might or might not work", so the specialty will be aimed specifically at increasing the power of these skills (not frequency, but power, since soldier's luck is for increasing their success rate, and the main skill of the race will be luck (legacy from ancestors)), also an increase in creature growth can be added, like army rally, only more collective:) for example, 5th upgrade of the main skill and let there be additional growth up to the 5th level..
Special skills will be such as Magic Shield (reduces damage using hero's mana), Knowledge of the Sea (grants the hero lightning and ice bolt spells; if the hero has sufficient knowledge, he can learn ring of cold and chain lightning). Storm Lightning (special attack dealing damage to several enemies, depends on hero level)

P.S. The idea was prompted by the naga race from Warcraft 3

Also wanted to insert a few pictures: