The Gateway was a pan-dimensional structure located at the mathematical boundary (where the co-ordinates were at zero) of E-Space and N-Space. It allowed for transference between Minkowski Space to other quantum states, access to non-Euclidean geometries, post-mathematic spatio-temporal co-ordiantes, and something Adric described as being "extra-universal." (PROSE: Cold Fusion) It was formed from the fabric of a Charged Vacuum Emboitment. (PROSE: Doctor Who and Warriors' Gate)
It appeared to be the remains of an old castle, possibly constructed by the Tharils as it was the centre of their empire. Inside the Gateway were mirrors the Tharils used to travel the universe, which lead to a black and white courtyard outside. They were guarded by the Gundan robots who walked the time winds where humans could not. There were three Gateways, the place, the arch, and the mirrors, and all the gateways were one.
Although it appeared as a free-standing structure in a white void, Romana II would later refer to it as a "planet," (PROSE: Blood Harvest) and the Fifth Doctor recognized a castle on Apollyon as the other side of the Gateway. (AUDIO: The Entropy Plague)