Normally the DF executable works like a terminal emulator, rendering the glyphs using GL or such. You can however run dwarf fortress in a console (ie, to run it over SSH) by changing the configuration file, and then it is a true text game.
BTW it doesn't use only ASCII but a larger subset of unicode. ASCII would be a very restricted set of characters to use.
It uses a mix of graphical tiles and text, it's not all just an array of unicode. They're sufficiently decoupled now that you can have most text rendered nicely using truetype fonts alongside whatever graphics set you're using.
BTW it doesn't use only ASCII but a larger subset of unicode. ASCII would be a very restricted set of characters to use.