Been a very long time since I did any spacecraft designs. General principles still apply though, remembering this is a self-contained, self-reliant unit, so essentially you need everything you'd have on a normal ship with a sizeable crew - they need to eat, sleep, stay clean (clothes and person; toilets as well), exercise, medical facilities, a means of controlling each system on the vessel, a means of communicating with other vessels and shore/planetary/orbital bases, a means of carrying out the vessel's primary and subsidiary functions, emergency escape facilities, emergency handling facilities (e.g. fire, flood, general damage control), a means of observing whatever is around them outside the vessel, and as you obviously already have, propulsive engines and fuel sources. This list is not exhaustive! And almost all this has to fit within the outer hull somehow.

Spherical and rounded hull shapes are a nightmare to design for. About 40 years ago, I drew out by hand a spherical free-moving spacecraft intended as a planetary defence vessel, one of a fleet, and that was horrendous, as (well, if you're me anyway) I had to keep checking how the outer circumference affected the internal space for different parts of every vertical level, and having to keep changing things because they'd no longer fit at the size intended, and so forth.

Unless there are strong reasons not to do so that make sense in-game, there's no physical necessity from our reality to have streamlined forms for spacecraft at all. Doesn't look so "Hollywood movie pretty", I grant you, but...

I've found the fractal settings can take a lot of playing around with to get right; commonly now, I either just go with the default drawing tool settings, or simply use the straight polygon options instead. Every now and again, there'll be a glut of problematic lines show up, simply because the fractal tool has added too many nodes in too small a space (possibly because I've needed to set two "click" points too near one another to get the shape right).

What you can do is just draw a straight guideline you need the fractal bumps to be behind first, and then as each new stretch of line comes up, if it looks wrong, just hit the spacebar key to try another fractal option for that part. (And keep hitting the spacebar, only to wish there was an "undo" option to go back to the one you've just spacebarred past, in my case...😏)

To my eye, the pockmarks are too regular in the image to work as weathering. Plus you may find that at different image resolutions, they don't always look the same anyway.

If you want the ziggurat to look more weathered, I'd suggest using fewer neatly-straight lines. Try drawing them instead with a new tool using a low-level fractal polygon, at least in places, and you might want to show some areas of greater damage if the structure has been standing for hundreds to a few thousand years (take a look at the Egyptian pyramids for ideas, for example, and, given the grey colouring, it may be worth looking at some of the more worn examples of diorite statues from the same region for ideas of how they look too). If it's been repaired in places, that might be an excuse to some some subtly different grey stonework over a few "stone/brick" equivalent areas.

Just a thought, but if there are four main staircases, shouldn't they each have a road/track leading to them (even if those are old, worn and hard to make out completely)?

The dunes aren't really working to me, because while everything else is seen top-down, they're viewed side-on. I'd be more inclined to experiment with some terrain drawing tools and textures instead, including trying some of the various darkness Solid bitmap fill options with a suitable bevel or lighted bevel effect to make them look sufficiently hilly and dune-like.

You could also have the canals running directly through the city (or connecting inside it in a variety of ways). Keeps the water flowing through, so it stays fresher, basically. From Sumerian archaeology (albeit very little's been done on this aspect), harbours in the cities could well have been placed a bit like market places - wherever there was convenient space and a couple of canals (or more) met, say. Water transport was just so vital for bulky, heavy or large cargoes there (e.g. nearly all timber had to be imported), it's arguable their canals and waterways were more important than the roads. Rulers seemed always to comment on having improved, or "built" (usually meaning something nearer "rebuilt" or "repaired" in many cases, given how many different rulers claimed to have "built" the same thing over time), with rather less emphasis on roadworks, beyond making them safe for travel (i.e. keeping the bandits away). That's probably too historically/archaeologically "real" for this setting, however 😁!

Happy I could help a little!

For a "proper" Sumerian settlement though, you need some canals passing through it, and maybe a bit less city planning 😉!