The Edge

The Edge of Existence was a symbolic, metaphysical place in the Doctor's mind where each incarnation went before regeneration.

It was represented to the Thirteenth Doctor by a barren cliff with a telephone pole. Here the Thirteenth Doctor interacted with the Guardians of the Edge, manifestations of her previous identities who maintained their own individual personalities while acting together as one being. They told her that she was determined to not have passed it yet, and that if she did, there would be no return. (TV: The Power of the Doctor)

Similar events
In a dream, the Fourth Doctor encountered his previous incarnations in a heavenly garden, which made him aware that his own regeneration was on the horizon. (PROSE: Into the Silent Land)

During the Seventh Doctor's battle with the Timewyrm, Ace entered the Doctor's mind, encountering various past Doctors in various guises, including the First, Third and Fourth Doctors. She discovered the Fifth Doctor had become symbolic of the Doctor's conscience and freed him from the confinement the current Doctor's persona had put him in so he could what he believed was necessary to stop the Timewyrm. (PROSE: Timewyrm: Revelation)

The Eighth Doctor found himself in a metaphysical place after he was stabbed and killed by his companion Charley Pollard. There he talked with previous three incarnations, who judged him on his death, before helping him to summon Zagreus to inhabit their body to ensure that the Doctor would not die for good. (AUDIO: Zagreus)

The Tenth Doctor encountered his next incarnation in a dream, assuring him that they would both be fine. (COMIC: To Sleep, Perchance to Scream)