No, I am not talking about the terrain sculpting process!
But when a building, or decoration like a statue, is built on a slope, instead of the item having a level foundation, and thus pointing vertically upwards, the item conforms to the slope of the terrain, meaning all your buildings (with some exceptions: see below) and decorations are on a lean. This is even true for (some of the!) trees you put down as decorations, although apparently not all.
The main exceptions are the “network” assets: walls and fences. For walls, this is great, because in-game they are sunk into the ground and do not conform to the terrain, so stand vertically: a good thing. Fences however look cray, because they simultaneously do not conform, and yet do not sink into the ground, meaning their slats can hang a ludicrous distance off the ground, and the posts reach high into the sky. They should be handled with non-conforming posts, like now, but conforming slats.
Other items should either flatten the ground when completed, so they don’t lean all other the place,or, more realistically, but no doubt much harder to model graphically, be able to sink and rise above the local microterrain as needed.
I currently only play on flat maps because I simply can’t stand the visuals of the hilly ones like alpine valleys: all the buildings leaning at crazy angles just breaks the immersion too much for me.