Wonder Boy in Monster Land

There are spots in the game whereby treasure will appear if you jump on them. To give you an idea of where to start, the first secret is located on the tree to the left of where you start your adventure. Some areas contain three secret coins, each of which count towards the total.

Unfortunately, there is no definate way to identify these spots, but you will probably stumble upon more than enough of them during your playthrough to unlock the trophy.