I personally can't comprehend building software with the waterfall method. I always do a reverse waterfall: build the fundamentals at the bottom. They are the non-negotiable "truths" of the problem, representing something close to the minimal complexity. Then, tie them together to work towards the relatively mushy end goal, with the abstractions being much more obvious as you work your way up, with a few "shifts" where higher level abstractions may pop out and collapse things into simpler pieces as you go up.

I would consider the GUI being one of those "truths".