From the Introduction: "This is the third product in the "Portals" series. The first two products, Portals of Torsh and Portals of Irontooth, deal with worlds in which the primary inhabitants were significantly different from those of a normal Advanced Dungeons & Dragons world. This product is slightly different in that the inhabitants are fairly normal, but the world itself is somewhat strange. Halfworld, the world of Portals of Twilight, always faces the same side toward the sun."