Hey,I am thinking,how did the Estharians cross the Salt Lake?
It was a Lake back there.
And how was the horizon bridge built?
The bridge have great columns that sits on the Sea floor and extends very far.
Here is the answer:
FH is not only a city but a ship working with different forms of energy.
The three mettalic floors enables them to build the bridge and to clean it.
It could even be that it has subaquatic floors or the factories along with the mayors house served to cover people underwater (supposing it works as a submarine).