Hmmm.... Yup. I would try a combination of stairs and plateaus. Did you check comments about if others worked this out and how? You may also be able to use a series of tiles instead of stairs. They discussed in the terrain video from EA before its release, and I actually did it on a lot I used terrain with - though it was a bit fidgety. Check out this video especially at about 54:48 into the video where they discuss this:
https://www.twitch.tv/videos/321249247@Sigzy05 Is it a Maxis Fave? I couldn't find it 'curated' and gave up searching through the uploads for this builder (kept not reaching the end and the same things were coming up), and when I search this title I get nothing. It may help me find it to know that.
@CaioSantos Can you give us a link to the gallery upload? Also, is it possible they realized the issue and took it down and are fixing the routability issue and will re-upload? Just some thoughts. It sure is beautiful! Weird question... if you look in build mode is the castle possibly just for looks and only the little houses are usable? Can you get into the houses?