Real-world observatory domes aren't "pure" spheres (nor are they transparent, but appreciate this is for a fantasy setting!), and are often made up of either individual smaller flat pieces - such as a geodesic dome - or curved, commonly into parts of a hemispherical gyre. Modern methods allow the construction of larger curved dome areas (using, e.g., fibreglass), but in all cases, there needs to be a segment of the dome that can open, either as a slot from apex to base of the dome, or where part of the dome can be removed/slid away, because the whole point of the dome is to protect the viewing equipment inside it from the weather, including temperature variations, both when in use and when not.

Consequently, the final red-roofed structure in your first posting's image is probably closer to a genuine observatory dome overall than anything else. You could even make the joins between segments more obvious, and you could add a rectangular panel for the viewing slit in the dome (because these don't narrow towards the dome's top - the instrument inside still needs the same viewing width for the slit to work). However, this really depends on what the observatory is used for - a transparent roof suggests something unusual inside, for instance.

Something you could try, based on the standard CD3 house roof options, is simply to increase the number of panels involved, since if you create more roof panels, the whole starts to look more rounded-circular, and loses its angular shape. Appreciate you're doing this in DD3 instead, but the principle holds - rotated copy by a specific small angle once you have your initial slim roof segment drawn. I've done that in CD3 drawings before now, to get a conical roof, for instance. If you're unsure what size of roof segment might work better, try some experiments using the CD3 drawing tools first.

Before you import the image file you want to trace to your CC3+ drawing, make sure you have a copy of the image file in the same folder as your CC3+ map. If you don't do this, you may well end up with a big red cross where your image file should be the next time you open it, because CC3+ can't find it any more.

There are other ways to avoid this happening, but keeping the files in the same folder is by far the easiest to tell you about quickly here.

I do this all the time, and it becomes second nature after only a short time; import, rescale, and get mapping!

Thanks @Ricko. In cases like this, I simply try to work with the original style as far as possible, and it is practical to "force" the Mike Schley cliffs to work as north-facing, even if it does mean quite a bit more effort. It would certainly be helpful to have some north-facing cliffs in this style already though!

For the central valley here, I simply added a Solid 10 polygon over the lower part to darken it a little. The effect is very subtle, but I found using the Solid 20 fill felt too obvious, whereas this seemed to help the effect just enough to fool the eye - to me, at least 😎.

Glad you got that sorted. Those are nice-looking floorplans!

Ellipses are fairly straightforward in CC3+. Just right-click on the Circle drawing tool icon - |CC2CIRP| - to call up the options list, and there's an Ellipse tool right there.

Spirals are a lot trickier though. Remy Monsen wrote a blog post about creating a macro to draw a spiral here, and although writing macros like this is pretty advanced stuff compared to simply drawing things and placing symbols using the program, they will let you automate much more sophisticated tasks with elegance. That blog post is the third in a short series of three on using macros, and you may find it useful to read through all of them (there are links at the end of that "spiral" post) if you're unfamiliar with how they're written before embarking on spiral design tests. And as usual, if you get stuck, just ask again here!

The other key element is the Color Key effect MUST be at the top of the effects list for that Sheet, or it doesn't work.