40d:Dwarf fortress mode
Fortress mode: where the game gets its name! This option is available from the main menu after creating a world. In fortress mode, you pick a location, then assign your seven initial dwarves some starting skills, equipment, provisions, and animals to bring along. After preparations are complete and your hardy explorers depart, they'll be faced with the fortress site you picked down to every detail, from geologically appropriate stone types to roaring waterfalls to ornery hippopotamuses.
Your view of the in game world is that of a multi-layered environment which you can move north, south, east, and west, as well as up and down. Dwarves are represented by little faces, rocks by blackness and open space by blueness. There is a command menu that lets you set commands that your dutiful dwarves will attempt to follow. The rest of this wiki is dedicated to helping you with these commands. If you need help starting out, see Your first fortress.
As an alpha version and sandbox game, there are few goals imposed upon the player by the programming. Survive, accumulate wealth, and build, build build! (Or at least try to make sure your inevitable death is entertaining). Some more specific challenges are available here, compiled by fun-loving players from all around: List of Fun Goals