There's a good description of the castle on Wikipedia.
Brilliant, the castle's website lets you explore the castle like a pick-a-path adventure. Enjoy the disorientating maze-like design (see Wikipedia) first-hand.
As for four seasons in one day though, Japan seems to get all four several times.
P.S. As a testament to just how un-vegetarian Japan is, I found more things I could eat tonight at a Brazilian meat-bar in Kobe (complete with waiter shouting 'Gorilla' and 'Anaconda'), than anywhere else since I got here.
Proof I was here. And some other guy too.
View from the top. Drafty as anything up there (only because the windows were open, if everything was shut-up I imagine it would be quite warm).
Not a great pic, but it's something different: Harakiri-maru (signposted as Suicide Corner). Probably not used though.
The square bit in the middle is a covered well.