Fearful Symmetry & the Cursed Prince

There are a total of 60 treasures, which are in the form of crosses, to obtain. Levels 2 through 20 each have a single treasure in them, which you have to collect with each of the 3 characters. Levels 21, 22 and 23 have the final treasures. Levels 21 through 25 don't unlock until you finish levels 1 through 20 with all 3 characters though. To see which levels have treasures, there will be a cross next to that level on the map. Once you collect the treasure and finish the level, the cross icon will be gone on the map. 

Refer to the achievement solution for each level to find a level solution that includes collecting the treasure in each level.