Doors play a large role in The Dark Tower Series as they allow for transportation between distant places or even between worlds. There are two types of Doors: magical ones left over from the Prim and man-made ones made by the Old Ones.
Magic Doors are the rarer form of Door found in Mid-World. They allow two-way transportation between two places and there are no ill effects associate with using them. The artificial Doors operate differently. They only allow for one-way transportation and are prone to deteriorate over the years. This deterioration can cause nausea if one uses these Doors. They can also stop working after long periods of use.
Examples of magic Doors are the ones that Roland Deschain found along the beach of the Western Sea and drew his ka-tet from. Man-made doors could be found in Fedic and Castle Discordia. Also, one of the most important doors was the Unfound Door, that Susannah Dean used to get back to an alternate New York City and also marked the Dark Tower.