During the challenge, I discovered something which I didn’t hear in the video, nor have I ever heard it in other tutorials (maybe it’s a newer feature?), even the “view”-menu does not mention this:
Num-9 flips the view!
So you know 1,3 and 7 select the front, right and top, and pressing ctrl with this selects the back, left and bottom? Apparently, if you are in one of these views, Num-9 immediately flips to the opposite one (i.e., right <=> left, front <=> back and top <=> bottom). If you are not in any of these 6 views, I think it just does a 360degree rotation around the z-axis.
If you are working in a specific view, and need to start working on the other side, instead of remembering if you were working in the “left”, “right”, “front” or “back” view (I find these easier to confuse than top/bottom), it seems easy enough to just push num-9 without thinking (or without looking in the corner to find the name of the view) 
