In an alternate timeline the time war didn't happen and the time lords survived. The Eighth Doctor in this alternate timeline was forced to regenerate and was exiled to Earth to Camelot in the same time period of King Arthur. In this new incarnation he was given a job as a jester in King Arthur's castle.
The Eighth Doctor was punished for vengefully eradicating the daleks after they threatened to attack planet Gallifrey. The time lords exiled him to Earth in a new body. For months he lived on the streets of Camelot in King Arthur's time. But then after The Doctor did a juggling act he was admired so much he was given a job as a jester in King Arthur's palace. People wondered who he was and it might have annoyed people for him constantly saying he was "The Doctor" so he adopted the alias Carlisle. The Doctor under the alias Carlisle noticed peculiar things in the castle such as one of the men in King Arthur's court saying "Join me Doctor, together we can rule the universe!" The man who had said that Sir Penn had been imprisoned twice. He had been a criminal, but for some reason King Arthur had let him join the Knights of The Round Table. The Doctor had not had good memory after his exile do he was unaware of what Sir Penn meant.
Wealth and Marriage
The Doctor later figured out that Sir Penn was the new incarnation of The Master, who had escaped from the Eye of Harmony, and stolen the body of a criminal. Sir Penn tried to kill King Arthur and take the throne but The Doctor defeated him by accidentally knocking him off the castle tower during a sword duel. The Master died without regenerating because the body he possessed was a human therefore with no regenerative cycle. Because of The Doctor's heroic deeds he was made rich. King Arthur built him a huge mansion near the village by the castle. The mansion was called "Carlisle Manor". During this time The Doctor had married the daughter of a rich widow and had, had children named George and Stella. But however The Doctor knew he was a time lord and could regenerate but he had forgotten that he couldn't age. So he figured out that humans age and die while he lives forever so the heart broken doctor forced himself to leave his family. He later figured out how to use the TARDIS again and went on adventures again like he had always done.