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[edit]
There are two secret caves inside the cliff at the far right of Maugham Desert. In order to get there use the teleporter in the pyramid to take you to the mountain area. Once there, head left until you reach the cliff edge. Fall off the edge while hugging the right wall (hold 
during the whole descent). You'll end up inside the hollow wall that leads to the first of the secret caves (this one has 2 units of Shield magic). Repeat the process by exiting and continuing to hold , hugging the wall, to reach the second secret, containing a 2 unit upgrade for your Power magic.
Pygmy areas[edit]
There are four 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 second secret area you can enter as a pygmy is in the ruins near Lilypad. From the start, walk to left, descend the ladder and go left. Here you'll find another heart container.
The third small secret area you can enter is actually inside the volcano. On the second drop, when you've unlocked the wall blocking the left section, walk to the far left lava pool (where the flame-caterpillar comes out) and jump in, against the wall. You'll end up in a secret room containing a chest with another heart container and a door back up to the middle section of that level.
The fourth is in Nightmare Castle. On the tracks, the lowest level has a secret door all the way to the left that will change Shion into a pygmy. If you proceed up a few levels, you will find a passage only accessible to a pygmy.
Bonus: while not strictly necessary to be a pygmy in order to get it, there is a treasure chest just before Shion reaches the ice gate which is broken by the bracelet. Once, he becomes a pygmy, he can jump across the gap to get the treasure chest and get quake magic. It's also possible to jump the gap with the legend boots or ceramic boots.
Secret door in the pyramid[edit]
Inside the pyramid there's a seemingly inaccessible door beside a chest, with no visible entrance. The door is hidden. Go right, near the wall (as shown in the picture) and press 
to enter the hidden door. The secret chest increases the MP maximum of all your spells - Fire Storm, Lightning, Earthquake (Explosion if playing the TG16 remake Dynastic Hero), Shield and Power. If you're missing any of these spells, they'll now be acquired.
Secret doors in the sea[edit]
There are 3 secret doors in the Monster World sea near Alsedo. They will all lead you to treasure chests with money.
Minor secrets[edit]
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.
Hidden Purapril Charmstone shop[edit]
At the bottom left of Purapril, there is a hidden shop covered in leaves and closed windows. If you have 1 Charmstone, he will buy it from you in return for an additional heart, if you have two or more he will offer to buy two in return for additional magic. You can repeat this until you have 14 hearts, and 12 of every magic except Return.
You should not need to buy hearts from him, as there's already enough in chests in the game to reach 14.
There's a Charmstone in the Pyramid, or they are 500,000 gold to buy.