Ok, so… I was building my cute little village, extremely happy due to the fact that now we can actually finish our buildings properly… (windows and doors included!)…
When I had this idea…
And this happened:
So, I was really excited it worked… I mean - IT REALLY WORKED… So I had another idea, and then… This happened:
My soldier watching over our lands on top of a tower… Wait, a tower?
Ok, so… On with the report!
You can build on top of other buildings. I made a quick search and didn’t find anything like that, but I’m sure some of us probably figured it out too!
Doing the tower, however, was the tricky part.
I had to build without Rooftop, wait for it to be finished and then create another floorplan on top of it. Cover all its area and then with the eraser tool, REMOVE the floor over the walls. Then I would raise walls and they’d fit perfectly over the building below.
For the internal ladders, AFTER raising walls, just erase some of the floor and add the ladders!
You can do it with buildings any size and shape, I guess… So, yeah, with some patience and hard work, you can build castles already!
CASTLES!
I just wouldn’t recommend it because… well, this is not supported yet, so, internal view gets messy. Big buildings would have whole sections hidden by the upper levels’ floors.
Oh, and to raise the walls, you’ll have to manually place ladders on every tile… Or else they won’t build up. Think of it as manually placing Scaffolding 
That’s it. All Hail the brave and inventive people of Mountainside!
I hope anyone find this piece of information useful 
I surely had fun doing it!
And as always, Thank You Radiant Team for this brilliant, lovely game!
It’s quite the opposite, however. I had some goblin trouble when I started, got hit by an early wave… Then I realized that the goblins seem to spawn on the edge of your explored territory, near the woods. I cleared all the woods up to the hill to the next level and now no goblins attack me 
Now we just wait for the first player to build a whole castle 
