So the planet isn't round. Has a giant plateau with gods on it. Olympus?
No one has been far enough to say it isn't round but yeah Olympus is the right idea.
Day and night depend on the Shadow the plateau makes? Then does it bear any resemblance to the tower of Druaga?
I guess it does at least partially. Hadn't noticed before.
So you had the Demi-Gods that drank the blood of the inventor, lets call him that because it sounds awesome, "inventor", and become powerful. But the other group fled to the ground where they lived in three rings?
Yes at least for now but they will one day get back up there.
Comfortable Western European late middle ages terrain, Nebatian desert like form with camel trade routes and strongholds a days' walk away and then the last bastion of hope to men in the outskirts?
I was generalising so do not worry they is more than enough space for all the forests and swamps etc you could want.
What is going on with the great beasts?
They're around but much less prominent. People are much more able to deal with them now. A lot of the story is metaphorical and about how people have learned to manipulate nature through the use of tools and agriculture.
Do they roam the countryside, terrorizing the populace but are held back by an order of mechanisms that harness the powers of nuclear physics to blow them apart?
When they are alone and theirs lots of us most of them are hardly an issue. There are a few big ones (time, and decay for example) that are massive problems. Most of the others can be managed since their intelligence is fairly low. But if you wander out into the wilds and storms or frost come for you're pretty screwed.
Also harness is used in a much more "low tech" sense. As in tie to farming equipment or making the wind birds work with your windmill.
The order is separate from that Medieval world and entrance is by birth or by being left there at their doorstep as a young child?
I vote for a Pantheon of patron Gods! One for each city! That would be awesome, just have the demi-gods compete against one another by bestowing grace on their people and then making them fight! Then you might have some of the cities unite against the reign of their gods and try to fight them off...
of course there's a pantheon being the blood line left by those who betrayed Serragish. As for a god for every city it's not exactly that simple. There are gods for all manner of things and a city or two might have their preferences. Then there are the people who still worship Serragish and see the other gods as being evil resulting in a spiritual boycott of sorts but admittedly their numbers and power are shrinking rapidly.
Maybe the Gods wield the power of those great beasts?
That is the source of their power yes, for this reason there are only gods of natural things, they have no power over things built by humans. For example there might be a water god but not a boat god. Other things like economics also don't have gods because that is again something unique to humans.
meaning that they are their minions?
Some of the gods do manipulate the beasts or try, tame them and use them as minions but the beasts have no loyalty to them or any sense or allegiance. In fact most of the beasts dislike the gods just as much as men.
So on one hand you have heavily armored mounted knights alongside men in metallic tank like vehicles, electrical weapons and on the other hand you have Demi-gods with various powers aided by those great beasts?
That sounds a bit advanced to be honest. Maybe once more is sorted out but no for now.
That help? Also what do you think of what I've done so far?