The category "worlds" includes universes, ages, and planets without any distiction.
These are the universes included in the series:
- The Earth, in at least three parallel dimensions:
- The Avatar's Earth (i.e. the homeworld of the Ultima players);
- Eodon (The Savage Empire);
- The Earth (and Mars) in Martian Dreams;
- Minax's Alternate Earth (Ultima II), that includes four ages: Earth (Pangea), Earth (1423 B.C.), Earth (1990 A.D.), Earth (2112 A.D. Aftermath); the 2112 A.D. age includes, in turn, the following planets: Mercury, Venus, one version of Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Planet X.
- Britannia, in at least two parallel dimensions:
- Sosaria/Britannia in the main series;
- Serpent Isle, separated from the rest of Britannia during the First Great Land Upheaval;
- Britannia in Ultima Online and its various facets (Felucca (facet), Trammel (facet), Lost Lands, Ilshenar, Tokuno Islands, Ter Mur);
- Caledonia, a slightly future Britannia (but still medieval) in Lord of Ultima.
- The Time of Legends (Ultima II).
- The Ethereal Void, including Malas (a facet of Ultima Online).
- Pagan (Ultima VIII).
- The Guardian's worlds from Ultima Underworld II:
- Balema, homeworld of Gorn (in The Quest and Ring Quest).
- Ashtalarea, homeworld of Seggallion (in Knights of Legend).
Furthermore, the Hallway of Worlds is an artificial passageway that is separate from any of the aforementioned universes.