So, what the heck is an underwater city going to be like? It's going to be built in three diminesions since everyone can swim up as well as laterally. It can be dug into the silt and ground, or be built alone an underwater cliff. Coral could be used as a building supply. It doesn't grow very fast naturally, but magic could be used to greatly increase its groth, shape it, and make it more stabl;e. There can be great kelp gardens and coral used as a natural barrier. THey can be lined with many poisonous creatures (the most poisonous of all creatures are underwater).