Monster World has many secrets that can be unraveled. There are lots of hidden doors and treasure-chests to be found by Shion.
Secret caves
There are 2 secret caves inside the high cliff at the end of the Maugham Desert. Use the teleporter in the pyramid to get to the mountain area. From here walk to the left until you've reached the end. Fall from the cliff and keep the D-button pressed to the right. Now you'll automatically enter the first secret cave with 2 shield elements, below this cave there's another secret area. You can enter this one the same way as you did before.
Pygmy areas
There a 2 secret places which you can enter after you're turned into a pygmy. The first one is in the pyramid. You can reach the pyramid by using the teleporter near Begonia. Once inside go to the room with the treasure chest and the secret door. Now you're able to walk to the left where you'll find a charmstone and lots of gold. There's also a heart here, just walk through the left wall in the room with the 4 doors. The other secret area you can enter as a pygmy is in the ruins near Lilypad. From the start, walk to left, decent the ladder and go left. Here you'll find another heart container.
Secret door in the pyramid
Inside the pyramid there's a seemingly inaccessible door beside a chest, with no visible entrance. The door is hidden. Go right next to
Secret doors in the sea
There are 3 secret doors in the Monster World sea near Alsedo. They will all lead you to treasure chests with money.
Minor secrets
There are lots of spots where you can find gold. For example on the tree trunk in the beginning of the game and left from the ladder near the tree at queen Eleanora's hideout. You can also try to jump near the arrows which point up.