This is what happens when you miss something that's not a building and then convert everything left to be a building:

In this case, it was a line used to show where the city walls should go. I drew the walls but forgot to delete that part of the line. When it converted the line to a building, it connected the two ends of the line to make a polygon. I used the "Change like draw tool" to change it to a dirt road, which resulted in this:

And then I deleted it, since it was putting in a road where no road ought to be. The same thing happened with part of the north part of the wall.

I fixed those two fields. I can't even find the light brown with furrows that I thought I used. Maybe I hallucinated them. ;-)

In fixing this, I noticed that the paved road on the left was accidentally on the ROADS sheet instead of the ROADS PAVED sheet. Fixed it after exporting this JPEG.

Here's a close-up of the fields. The two empty ones were furrowed brown fields. Not sure why those fills disappeared. (I just checked, and they now have a fill style of Hollow. Hmmm.)

Following my test runs with the village of Greenwood Falls (population 920) and Redway Hill Township (population ~3,500), here's my attempt at a major city: Wolfwell Falls (population ~55,000).

This is what Watabou gave me to work with:

And this is what I came up with:

Here's a close-up of the Castle District, which I figure is a bit more upscale.

(I kind of messed up the river there. I when I did the "Change like drawing tool" command, I accidentally wasn't on the River/Water sheet, and I think when I fixed it I messed up the top layer or something.)

The nicer neighborhoods have a lot more greenery. Most neighborhoods have more dirt than greenery, and the area by the docks is muddier.

As predicted in the Redway Hill Towship thread, the longest part was redrawing the secondary roads. According to the Info: Count tool, there are 59 paved roads and 948 unpaved roads! I hope I successfully converted and/or deleted-and-redrew all of the roads. I had to do all of that, and also create a handful of special buildings, before being able to convert everything left to be the regular buildings. If you see any really weird buildings, it might be something I didn't properly convert to whatever it was supposed to be before turning it into a building.

I should draw a lot more fields -- the ones on the north side of the city won't be enough to sustain a city of 55,000 people! (Also, I just noticed that I call it "Wolfwell Hall" on the map instead of Wolfwell Falls. Gotta fix that.)

Okay, here's a larger town. I kept getting interruptions, but I'd say it took me about two hours. This time I imported the trees, but I must have been doing something wrong when I changed like draw tool, because I was just getting the outlines. I ended up manually adding the trees using the draw tool.

With this map and my previous ones, I changed the buildings in small batches, by blocks or neighborhoods. I did that in part because I kept missing roads, and then adding them as I was going neighborhood by neighborhood. When I tackle a big city, I will try it where I do everything else first, eliminate extraneous non-building import lines, and then select everything left to convert into buildings.

This is what Watabou gave me:

And this is what I turned it into: