While extraordinary to us, an underwater settlement might be commonplace on your world, if a water dwelling species exists to construct them. Are there dry areas or air pockets allowing land species to reside there safely? Or be imprisoned because there’s no way to reach the surface? Perhaps magic or technological portals allow people to enter or leave such a city. Think about what sort of industry and skills a water dwelling species might acquire if they not only have water-filled areas but dry ones, assuming they can move on land. Might they craft a large enough space to practice with weapons they’d need skills for on land?
We’ve seen cities in Star Wars, and rock formations in Avatar (italics), both suspended in the air. We can use magic, technology, or unexplained physics to do the same. Aside from this floating aspect, these settlements otherwise differ little from more ordinary communities. However, we should think about what opportunities are afforded. A flying species might be prone to inventing such places or find them very attractive. There might be few predators except the flying kind, making life safe. While no city walls will exist, what kind of aerial fortifications might be needed? Trade might be quite difficult without ships, large flying animals, or other means to transport resources. It may also be at extraordinary risk of crashing to the ground, which seems an obvious sabotage focus for enemies. How does this place protect itself from hurricanes, tornados, or strong storms? The obvious answer is that it’s only built somewhere that doesn’t experience these.
On Earth, we have Venice, which is built largely on stilts, but the impression is still of floating on water. This does not require magic or advanced technology, but we can invent a place that genuinely floats on the sea. A water dwelling species might find this accommodating and even be the ones to invent it. The setting can be a place where land and water species can interact more easily. This must occur where significant waves are rare if not unheard of, so look for a lagoon or otherwise protected inlet in waterways like bays or sounds. If there are large sea monsters that could easily wreck the place, then it won’t exist, but if there’s a new sea monster never considered before, we can have fun destroying the settlement in a story.
Other Unusual Homes
A species might build homes inside hills like the Hobbits of The Lord of the Rings. Elaborate mountain homes are a staple of fantasy dwarves. A flying species could build small homes inside enormous trees, but with no way to reach the ground so that predators, including other species, can’t access them.