Those were done with the pen tool - if you don't click and drag it lays in a straight line. In this case a number of clicks will give you a shape with straight sides. Many clicks will give you lots and lots of buildings. The advantage of doing this as paths is that it's vector and therefore easily editable down the line if you want to move things around.

The smaller buildings were done by using a rectangular brush, setting the spacing to around 200% and adding size and roundness jitter. Also set the orientation to 'direction' so that they follow your en strokes. This lets you draw in small rectangular buildings really quickly. I'm not able to provide much more detail right now, but does that help? I've got a houses tutorial in the works, and I'll make sure to bump up the priority a little.