I'd say a lot of patience and clever use of selection paths. I say selection paths, because doing it all with the eraser brush by hand isn't very efficient and would be ridiculous. (When possible, work smarter not harder.) And don't forget there's always quickmask mode for those funny shaped spots. For buildings I'd either start with polygon lasso or pen followed by path to selection. Plugins may make it go easier/faster, but if you know enough of what you're doing you don't always need them.

Now the question is it done with masks, or are they erasing? (Not that there's much difference in the end result, but more a curiousity about workflow approach.)

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.