Yeah, that's what I though. The corners shouldn't be peeked because it is too square. But, if you add smooth, the shape would look strange. You would have to smooth it and ass some splines to make the corners look better.
But it does look good! I know that you aren't very experimented but you kept the "cartoon style" from the picture.
and wasn't there supposed to be a river surrounding the castle?
Also, I'm starting my final exams in 2 weeks so, in 3 weeks, I will be done. I'll work on mario then.