The primary Fortress mode and the Legends mode will be accessible right away, with the Adventure and Arena modes from the original game arriving later. Even said, Bay 12 is far from finished, with the studio planning significant adjustments to the map system as well as procedurally generated mythologies and magic systems. According to a press release, the project has “no conclusion in sight.”
If none of that appeals to you, you can always download the original version of Dwarf Fortress in all its ASCII splendor from Bay 12’s website for free. Until now, fans have contributed to the construction of Bay 12 and the game.