When the Doctor and Clara Oswald arrived in Christmas, they were soon taken to the Clock Tower where a crack in time, the last crack in time, appeared. This was the crack from which the Time Lords were sending out the Question in order to return after being sealed away by the Doctor in the Last Great Time War. The entire town was enveloped by a truth field emitting from the same crack so that the Doctor would have to answer honestly the one question for which he alone knew the answer. Soon after sending Clara back home with his TARDIS, the Doctor took residence in the Clock Tower, which became his home, headquarters and base of operations once the Siege of Trenzalore began. For 900 years the Doctor live here until Clara's second return, continuously defending the town and the crack, yet refusing to release the Time Lords since he knew that if they were released, the War would begin anew.
After Clara's return, a dying Doctor made his final stand against the remaining Dalek army at the top of the tower, where the bell hung. Meanwhile Clara approached the crack and begged the Time Lords to help the Doctor, which seemingly sealed the crack. An even bigger crack opened in the sky over the Clock Tower, from which the Time Lords granted the Doctor a brand-new regeneration cycle before the crack vanished once more. Filled with energy, the Doctor used the incredibly destructive blast from his thirteenth regeneration to obliterate the Dalek fleet. Afterwards, Clara and the Doctor returned to the TARDIS and continued their travels after the Doctor's regeneration into the Twelfth Doctor. (TV: The Time of the Doctor)