"Test of Time included an exact replica of the old Civilization II campaign, with only a few minor changes. Cosmetically, however, it looked quite different, with all-new art and animated units. Test of Time also offered the option to undertake the Extended Original campaign, which was identical to the Original except that the "pink" slot for a civilization was occupied by aliens on Alpha Centauri. When one built the Alpha Centauri spaceship, instead of ending the game, it unlocked a secondary tech tree and allowed the player to battle the Centaurians.
Test of Time also included a fantasy world modelled after Norse mythology. The four maps of the Fantasy Campaign are the Surface World, the Underground World, the Cloud World, and the Undersea World. The seven species included each started on a different level. The central difference between Midgard and the Fantasy Campaign are the scripted events and premade maps for Midgard.
Source: Wikipedia, "Civilization_II:_Test_of_Time," available under the CC-BY-SA License.