Under his roof, the king stares into a drop of water containing his greatest treasure. Just kidding, the king has way more treasure than that. The drop of water is just reflecting what's around him: mounds and mounds of treasure, stacked all the way up to the roof.
All of this treasure is kept in the king's home (ほう). He can't let other people see how much treasure he has, or they'll get mad, or worse(!) ask him to share some with his people. Yuck. The treasure stays at home.