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Like Lord_Kane said, Anarchy levels are Multiplayer-specific. Basically anarchy is a mode where there are no robots and all the players are just fighting each other. I believe some older versions of Descent (and/or level editors) wouldn't allow a level to be made without a reactor, so many anarchy levels have reactors even though they're never used (and they may not have exits).
So Descent Series is coming to GOG.com soon again? Thanks.