I've searched through a lot of merpeople related questions on this site and didn't see anything that related particularly to me as most questions have to do with underwater combat or dwellings on the ground.
My species is a race of humanoid axolotls (that means legs and feet like people) who are perfectly capable of living on land. They do not have access to land though. Their cities are built on the trunks of massive "balloons" suspended in water. These balloons are full of oxygen with several inch thick membranes on each balloon. The spheres that contains the gas can range in size from a 3 meter to 20 meter diameter. The stalks reach up from the very bottom of the ocean, nearly 500 meters below, to from a few meters to a few feet below the water surface.
For extra clarity I'm going to restate what's so bad about the balloons getting popped. Cities are built out of a sort of underwater adobe and plastered on the trunks of these massive balloons midway or near the surface of the sea. If one were to pop it would soon fall over from the weight of the city on it's side. Obviously that's not good for my inhabitants or nearby balloon cities that might be hit.
So my question is: In the event of humans waging war on my peaceful civilization how would I protect against my balloons getting popped or from another form of attack from above?
The technology level is that of earth 1000 AD. My merpersons can swim at 20mph. Magic is rare if not nonexistent and most nearby countries have access to metallurgy and gunpowder and can launch projectiles at their enemies. The merfolk can tame things like dolphins but many do not have any creatures as they're difficult to provide for with the dense population zones around the balloons' stalks.