Here is the full list of all 20 Castle Of Pixel Skulls achievements.
Complete level 1 without dying
Complete level 2 without dying
Complete level 3 without dying
Complete level 4 without dying
Complete level 5 without dying
Complete level 6 without dying
Complete level 7 without dying
Complete level 8 without dying
Complete level 9 without dying
Complete level 10 without dying
Complete level 11 without dying
Complete level 12 without dying
Complete level 13 without dying
Complete level 14 without dying
Complete level 15 without dying
Complete level 16 without dying
Complete level 17 without dying
Complete level 18 without dying
Complete level 19 without dying
Complete level 20 without dying