They could be playing alongside you as jolly partners, or against you as wicked adversaries, or even above you as capricious gods creating miracles and spells. Second, you and the other players can add or modify rooms, as well as invent new and magical objects. So you make one, just by typing in its description. In short, the players invent the world as they live in it. The game is to create a cooler world than you had yesterday.
MUDs are very much about who is in the same place at the same time, and how they interact.
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