Secrets are found by opening chests that are located through out each level. The chests are not always visible some are hidden. The following is a list of how many secrets are found in each level.

Level 1: 4
Level 2: 11
Level 3: 6
Level 4: 6
Level 5: 3
Level 6: 6
Level 7: 5
Level 8: 0
Level 9: 16
Level 10: 17
Level 11: 3
Level 12: 5
Level 13: 5
Level 14: 4