Now, it is obviously not fully set in the past sense of anime. Clearly the characters look more of today while the art style is more from the past with a modern tint to it. Which actually works very well.
Now, I do have to state clearly that the story starts out rather confusing as it doesn't tell you what is going on, nor at the end you have all answers. It is also highly recommended for a audience that is a bit older, 100% now for small kids as they won't understand too much of it nor get much of the plot. So I would advice 16 years and older as ballpark idea.
Without telling anything about the spoilers, I can't really state much. There are so many people in the review section that gave up far too early on this one that I really have to state they are fools for doing so. By only seeing a few episodes and not finishing you will do yourself and this anime a huge disservice. I can't state enough that this could well be one of the big anime names as cult classics. I highly recommend to see everything and I for sure do recommend giving it a fair chance.
The second season will be very interesting as the end of season 1 ends up quite interesting. I wonder where they will take this one from here!
