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In reality, you can come up with a great many races. It all depends on what you need: a good Castle and well-known monsters, or your own invented race?
If it's the first, you could create something like Tauren, Orcs (although it's very difficult to wipe out an entire race), Sith, etc.
But if it's the second, you could create an underwater world: the Castle stands only on the water or a shoal, and the inhabitants are various types of aquatic creatures.
For example:
1. Mermaid (as always) - a melee warrior with the ability to throw a web and bind to a location.
2. Leshy - spits through a tube with a poisoning effect (but make the poison different from that of the Assassins).
3. Flying fish - not a very strong flying unit with a low health level, high defense, and immunity to magic.
4. Underground Lord - a warrior who does something like teleportation, but only underground, with the ability to burrow, doubling his defense.
5. Turtle with spikes on its shell - a warrior with the ability to shoot its spikes at nearby warriors. If attacked in melee, the attacker receives 1/5 of the damage they inflict. +15% protection from arrows and magic.
6. Water Lord - a mage, resembling a water elemental, using ice spells.
7. Underwater Serpent - a long snake (like a Chinese dragon) with the ability to jump - overcomes any distance and attacks the one in direct sight (at will), dealing damage multiplied by a coefficient that depends on the distance.