I think part of the issue your first building was facing may have stemmed from the number of adjacent “posts” in the walls (the brown pillars)–it seemed that many people had issues with structures like that previously. I believe most of the bugs were fixed, but it’s possible some still persist.
If you’re trying to make a rounded structure, it might be easier to draw the curves using the Slabs (free-drawing) tool, and then make walls for where you’d like actual windows and doors (note, this requires a minimum length, I believe it’s a wall voxel on either side of where you’d like it–4 for a window, and 5 for a door). It’s a pain, but that also means you’d have to draw the roof yourself. It’s daunting at first, but I’ve gotten quite used to it for my own builds (see Atralane’s Antics, where I’ve had to do it here and there).
Although, now that I think of it, another example of rounded buildings that did work is @micheal_handy76_mh’s Hearthean Theater back from…well, I actually don’t remember which Alpha. Maybe 8 or 9? Regardless, the structure should work. Maybe it has to do with the current unstable version or the way the top floor is in more slightly.
A rule of thumb for making structures I follow is always make a “normal” building first–draw the floors, raise the walls, place the roofs. Then start to add things like windows and doors (I don’t add furniture or lights to the blueprint, it’s still wonky). Afterwards, feel free to cut a hole in the floors for future stairwells or awnings. But make it after the building’s done! Otherwise, your worker’s get confused.
Another final little point (which you might have already learned) is that the building engine struggles with wide to narrow shapes; think a pine tree, for example. Going from a narrow/smaller shape to larger works fine, as the workers build another set of scaffolding once the inner part’s done. However, they have issues with building scaffolding within the base of a structure. If you look at the Diluvium City project of mine, you can see this in action: the big pillars of the Shrine had to be built by manually placing ladders, as the scaffolding only encircled the connecting floor around them (they were all one structure). Until the game can make things in “layers”, the best option is to split them into separate constructions so you don’t have to babysit the workers.
Hopefully some of these points are useful! I wasn’t sure how much of this was already known, but I figured I’d shared some of our experiences so far. Next build, I’ll try to experiment with circular structures and see if I can narrow down some of the issues.