A hexagon level is a feature which marks a level as a "Hard Level" in game. Notice that, the "hard" label may not be the same as the difficulty rated by this wiki. They are shown as skulls in the mobile version.
There is no special bonus for clearing a hexagon level in most situations. An exception is when hexagon levels are completed to fill the Piggy Bank.
Note that a level may change from normal to hexagon or vice versa. This generally happens when a level is redesigned, but sometimes, it happens without a redesigning. All hexagons may be removed or added.
Hexagons do not necessarily reflect the difficulty of a level. A level is considered a "hard" level if King thinks that it is hard, not the wiki.
It is possible that King will track the number and ratios of successes and failures relative to other levels. Some early levels with a higher degree of challenge may also be determined as hexagonal.
At a given time, the set of hexagon levels in web may not be the same as the set of skull levels in mobile excluding recently released web-only levels (for example Level A is a hexagon only in flash, but exists as a non-hexagon in HTML5). Hexagon levels in web are usually updated prior to HTML5.
In a mobile update in around February 2017, thunderstorm levels are implemented. They mark levels as "super hard levels." They are represented as skulls, but in addition to that, the level button is now dark blue hiding the level type, and a dark blue thundercloud hovers just over it. In 28 June 2017 the super hard levels released in Facebook with HTML5.