The Tower of Judgement was a prison on the planet on which Gavin XXIII was king.
Gavin's "evil" brother Conrad was imprisoned in the tower after being overthrown by a resistance led by the Second Doctor and Jamie McCrimmon. The natives of the planet intended to build a statue of their king to block their view of the tower.
When the Doctor and Jamie visited a nearly identical planet, the tower, in which they presumed Gavin had been imprisoned, had been destroyed. (PROSE: Twin Piques)