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)

Behind the scenes
Paul Magrs' 2012 fan fiction story The Runaway HiFi  states that the Second Doctor's trial during The War Games took place in the Tower of Canonicity. A reprint of the story appeared in the Obverse Books 2018 charity anthologyA Second Target for Tommy.