When starting a new game you have following options:
Dungeon[]
Choose the dungeon you want to play. Currently there is only one dungeon ("Grimrock") but expansions and user-made content will be added here.
Difficulty[]
Scales monsters damage, speed and aggressiveness in combat. This setting can't be changed after starting the game. The difficulties are Easy, Normal, and Hard. Completing the game on Hard difficulty earns the Hard Boiled achievement.
Create Characters[]
You can use this option to customize the characters of your party instead of playing with the predetermined ones. You can choose between four races and three classes, invest Skill Points into the character's abilities, and add up to two traits.
Toorum Mode[]
After earning the Buddies With Toorum achievement, it is possible to play through the game as Toorum. To do this, start a new game with character creation on. Change the first prisoner's name to "Toorum" (case sensitive; "toorum" will not work), making no other changes and adding no other prisoners, and press enter. You will begin a new game with a party consisting only of Toorum, who carries three traits and a unique class, and begins with Stat and Skill Points already invested. Additionally, Toorum's Notes will not appear throughout the game when playing as Toorum, making it impossible to achieve 100% completion on a Toorum playthrough.
It is also possible to play as Toorum without getting the achievement. To do this, go to the Legend of Grimrock save folder(/My Documents/Almost Human/Legend of Grimrock/) and create a text file named "toorum_lives.txt". The file can be empty.
Old School Mode[]
Turns off auto-mapping completely. Arm yourself with a stack of grid paper and pencils and be prepared to get lost in the dungeons! Beware, you can't change this setting after starting the game. Completing the game in this mode earns the Doin' It Old School achievement.
Ironman Mode[]
Introduced in Legend of Grimrock 2, in this mode, the game can only be saved at a Crystal of Life. This option can't be changed after starting the game.
Single-Use Crystals[]
Also introduced in the sequel, this mode does exactly what its name says: each Crystal of Life can only be used once. This option can't be changed after starting the game.
Insane Ironman[]
This refers to turning on both Ironman Mode and Single-Use Crystals—as a result, the game can only be saved by using one of the Crystals of Life, meaning that the game can be saved at most twenty times. Completing the game with these modes on earns the Insane Ironman achievement.