Towers of Canonicity and Likelihood

The Towers of Canonicity and Likelihood were twin towers which "lorded over" the northern part of the Capitol. According to Daedalus, they were the structures from where the Time Lords watched all of time and space. (PROSE: The Blue Angel) They were noted for being dimensionally transcendental. (PROSE: The Gallifrey Chronicles)

When the Healer ordered Christina Cwej to transport her into the Houseworld's early history, they unexpectedly found themselves in a rose field scattered with huts. The Healer expressed doubts that this was the right place, as she could not see the Citadel nor "the glittering lights of the Towers of Canonicity". (PROSE: Rebel Rebel)

Behind the scenes
Paul Magrs' 2012 fan fiction story The Runaway HiFi  states that the Time Lord courtroom seen in The War Games was in the Tower of Canonicity. A reprint of the story appeared in the Obverse Books 2018 charity anthology A Second Target for Tommy.