I've seen similar maps from 100+ years ago of my hometown in the UK:

http://en.wikipedia.org/wiki/File:Birmingham_in_1886.jpg

And of London:

http://www.garwood-voigt.com/catalogues/H242051LondonBrewer....

It's amazing how our idea of what is unachievable by hand has been skewed by computer graphics!

Not only can they build rendered cities at breakneck pace, they can do it for the real thing:

http://www.businessinsider.com/shanghai-1990-vs-2010-2010-6

Actually that's in fact possible. The models must be done by hand for landmarks, the rest may be repetitively. Similar efforts has been done by OSS games (http://wiki.openttd.org/City_Buildings_%28New_Graphics%29)

Obviously this is not what was done here, but how difficult is it to take a satellite photo from the side, instead of from directly above?

Not very. Most photos are taken from some angle (it's easier to reorient a satellite than to reposition its orbit). Course, you get the most quality when shooting close to straight down.