I would like to start off by thanking you for making the video, it was both helpful and really cool to watch! (I'm a sucker for time lapses)
It definitely is different from what I'm used to building, and because the skill is so much higher than what I originally accustomed to, I wound up a tad bit confused.
What I usually do is cut out the terrain, so my structure is definite; but here it looks like the castle was built to accommodate the terrain. Can I do what you did if I intend to furnish and fill in the interior of my castle? (It doesn't look like you filled it in from the video, and if you did then I apologize)
Thanks again, Mr Clavet!