A Hard Level is a level on Candy Crush Saga that is considered difficult by the majority of players. Levels can be hard for a whole mass of reasons. Any level type has the potential to have hard levels.
What Makes a Hard Level
Blocker - Blockers can make a level hard by blocking off jellies or by stopping ingredients falling to the bottom. A blocker with many layers can make a level hard by the sheer number of hits needed. Examples of this are levels 147, 181 and 290.
Chocolate Fountain - A chocolate fountain can make any level much harder, as they are unbreakable and never stop producing Chocolate. This will make a level much harder if they are in awkward positions. Examples of this are levels 167 and 213.
Difficult Objectives - Sometimes Objective Levels can be hard due to the sheer difficulty of the order collection, for example a very hard-to-obtain special candy combo, such as mixing two Colour Bombs. Examples of this are levels 140, 215 , and 435 .
Candy Colours - A level can have up to six candy colours. With six colours on the board, the candies are much more mixed up and matches are harder to make, as are special candies, making a level significantly harder than it would be if there were five or four colours. Examples of this are levels 140 and 347.
Misplaced ingredients - In ingredients drop levels, you have to drop the ingredients where the green arrow are, but sometimes, the ingredients are just under a dead space, where an ingredient can't fall, it is really hard to move a stuck ingredient. Exemple of this are levels 117 and 315.
Too many ingredients - In Ingredient Drop Levels, the ingredients are generated in function of how many moves you did, but where they are too many ingredients needed, when you run out of moves considering you didn't drop any ingredients, not all ingredients are present, forcing you to drop all the ingredients in the board to make a new one after one move. And it is really hard. Exemple of this are levels 30 and 325.
Many levels in the game can be considered hard.