Free RPG Dungeon Crawl Stone Soup is one of a class of games known as roguelikes, after Rogue, the first such game. The ancient legacy is apparent. Although this open-source game has a graphics mode, it can also be played in straight ASCII, where your character is an "@" and the monsters are various letters or symbols. The "tiled mode" provides simple 2-D graphics.
The goal of Dungeon Crawl Stone Soup is simple: Descend into the immense and randomly created dungeon, battling monsters, avoiding traps, and becoming more powerful, until you finally find the magic amulet and escape. (If random dungeons and magic items and unusual things to find sounds familiar, it may be because Diablo took a lot of roguelike concepts, then ramped up the graphics quality and ramped down the complexity.) Familiar it may be, but it's is hardly easy. There are countless ways to die (a common acronym in roguelike discussions is "YASD": Yet Another Stupid Death), and many things to discover. Each level can hold surprises and each game is different. Magic items are also randomized, and you need to test items to find what they do. The first item I found in my first game was a cursed ring of teleportation that could not be removed and would randomly transport me across the dungeon.
Oh, there are no tap-backs in Dungeon Crawl Stone Soup. Your progress is saved only when you exit the game. Once a character is dead, he/she/it is dead, and you have to start over with a new character.
Character creation is simple. Pick a species (race) and background (class) and get going. Most of your characters will die long before getting more than a three or four levels into Dungeon Crawl Stone Soup. Each class and race plays differently, and traits such as religion will impact gameplay, as different gods reward or condemn different actions.
There are hundreds of small details in the game, from food rotting over time to acid corroding your items; all of the processing power not dedicated to fancy graphics instead goes into making the underlying game engine extremely deep. Not "Dwarf Fortress" deep, mind you, but also a lot more approachable.
Dungeon Crawl Stone Soup adds many new twists to dungeon crawling, removing some of the tedium of other roguelikes without removing the challenge. Inventory management has a clean and easy-to-use interface. Skills auto-improve as you use them. "Trap Doors" provide one-way access up or down levels, whereas stairs can be used in both directions. There's an option to have the program automatically explore the level for you, moving your character in a pattern of exploration, and stopping if you draw near to a trap, monster, or other item of interest. Likewise, there's an automated process to retrace your steps back to an earlier level--no more wondering where you saw the stairs. You must still battle any monsters you encounter, of course.
In addition to the purely random levels, DCSS has "hand crafted" levels or areas that hold special challenges, opportunities, or rewards. Finding these unusual zones is one of the great joys of this type of game.
Dungeon Crawl Stone Soup is free and well-supported with an active community and constant development. In fact, the "Stone Soup" name derives from the many people who have added something to the mix. There's an extensive wiki that reveals many secrets of the game, but it's a lot more fun to stumble on them on your own. If you're a roguelike fan, you will like Dungeon Crawl Stone Soup. If' you've never played a roguelike, the well-done tutorials and hints make this the one to try.