I've got a species that lives in the Southern Ocean along the Antarctic coast, and they are basically a sentient crossbreed of Orca whales and elephant seals. They are omnivores, have opposeable thumbs, a level of technology comparable with our own, and have lived in the same place for many centuries, at a depth of a couple hundred meters on the sea floor. They are mammals and breathe air, but sleep for very short periods so I don't think their houses all need to be filled with air, per se. They are definitely not living under domes.
What would their cities look like? I have looked at construction materials from this question, and settled with quarried stone as their primary material.
I initially thought to make the city very tall and multi-levelled, with the highest levels reserved for hydroponic farms, but if you build tall towers then the currents may push them over, so I'm guessing that's not ideal.
What sorts of city lay-out would you get with a species that lives in the water? E.g., would their doors be on the top of their houses?