Every level has a series of Enigma codebooks, gathering all of the pieces of a codebook unlocks a cypher which, when solved, unlocks a new game mode. The cyphers and their unlocks are as follows:
This unlocks 999 mode, giving the player 999 health and unlimited ammo, but also substantially increases enemy difficulty.
This unlocks 'Walk in the Park' mode, which removes all of the HUD elements from the game.
This unlocks 'Hardcore' mode, which removes all health and armor pickups from the game.
This unlocks a permadeath mode. In this mode, the player has only one life and must start over from the very beginning if killed.