The city of Merfolks was huge, deeper, so deep that no human could reach the city gates even if they swam the deepest. At one point, they would be engulfed by extreme darkness, so much so that they would not want to cross the threshold. Even if some do pass through that area, they would immediately be killed by the gatekeepers standing in the dark.
Yes, there were two gates to enter the city, one was out in the open while the other was standing in the dark, not being able to be seen by any humans. Once they encounter these humans, they won't be left alive anymore.
In short, no one other than regular fishes could enter this place protected dearly by several powerful beings all across the sea.
Ever since being discarded from the surface of the Earth, these people had nowhere to go nd thus, they decided to stay here and find other small fish races beneath the surface to guard them at all times.
