Here you can see quite well, how the outline operation from a few minutes ago has helped us to create sidewalks and little alleyways to the backyards that were intentionally left open.
Since "select tops" was active in the preceeding greeble modifier, we can now easily alter all "roofs" of our buildings to use ID 4 (which is the dedicated roof material).
Afterwards you should deselect all smaller faces, because they will look unrealistic when we farther going to extrude them or even add another greeble level. As a rule of thumb you should leave only those faces selected, which are reasonably large and nearly quadratic.
Only faces which are now left over will receive an additional "floor" by extruding them. I thought I'd lower them a bit before doing this, so that the overall size increase would be counterbalanced.
You can ditch this step if you want. It sure isn't essential.
Assign material ID 2 (= 2nd floor material) and extrude the faces. You could also use Greeble instead of an extrusion, but it splits up some of the faces, and that won't make them suitable for the next step.
Now we are adding the actual building tops to our sub-selection by adding our 3rd greeble modifier for today. :-)
This greeble step can have ID 2 or 4 - you decide. We are again only creating "panels", but this time they act as hipped roofs by using some height and an increased taper value. I thought I'd assign Material ID 2 to it, and apply the roof material to the top faces only
As a last step we can now add some "widgets". Don't ask me, why I didn't do this in the previous step already... guess I'm getting old. But then again: having a separate modifier in the stack for each step makes it all easier to reproduce. ;-)
This layer is supposed to add chimneys, air conditioning units, maybe antennas or whatever type of clutter you find on roofs of urban buildings. In order to save some polygon budget I was only using the simple box type widget.
That's nearly all there is about the modeling part of our ground level.
As mentioned at the beginning, you will have to repeat this procedure for generating at least a few unique "street maps", so that you end up with enough tiles for making up a much larger "downtown area".
Lazy as I am, I just cloned the existing city quarter several times to have a preliminary surrounding.
Of course you can also quickly alter the face of these "city quarters" by mirroring or rotating them, deleting certain building blocks and place some of the individual skyscraper models there. We are going to do this in part 2 and 3 of the tutorial, which I hope to get done yet this year. :-)