Inca-flavored setting, though with dwarves around there’s iron enough that I don’t have to do a bunch of work changing stuff 
The civilized world is here above the clouds, below are monsters and the corpses of dead civilizations. And loot. But mostly monsters.
Dude that is awesome!
Stay cool 
Beautiful (I want to run a campaign in that world now)
Sweet!
That’s beautiful! Just by looking at it, I can think of hundreds of aventures to play!
Are those clouds natural? Why are there dead civilizations below? Are they “cataclysmic clouds”?
If you asked in a game, I’d say “that’s a good question! You should ask an NPC about that”
Which as we all know is code for “hell if I know, I’ll figure it out later” 
The way I’ve played it so far they act like normal fog, but they seem to be connected to a lot more monsters hanging around. Trolls love it, of course - though I did have a player debate a troll until the sun came up and stoned him, mostly because it was cool. I suppose, since he was playing a paladin and obviously an Inca-inspired setting necessitates sun worship that that was divine providence
In all seriousness, the clouds/fog act mostly as a divider between the realm of men and the realm of the untamed and unknown. What’s below the sea of clouds is hidden from the life-giving sun, and thus partway into the land of the dead. The bones of ancient civilisations may be the remains of a time when the sun was stronger and the clouds lower - or it may be that they are the abandoned kingdoms of the dead. They also lend themselves to easy descriptions, as everything below can be describes as slick with condensation, covered in mold or infested with fungi!
… and how do they get their water? 
But this is just… how can I say? This is completly AMAZING!
Good question, let’s see…
All on the mountain know to fear the dark days, when strands of cloud rise toward the sky bringing monsoon rains, and monsters sneaking through the streets. During those times people hide behind barred doors and shuttered windows, and pray for the sun.
Waterboys must be badass.
Stay cool
You dip a bucket down into the clouds and run the contents through a cloudmill.
I’ll follow it up with an industrial revolution version where cloud textile mills are big business