The Prison Towers are a location in Chrono Trigger. In 1000 AD, they appear as a section of Guardia Castle. After the rescue of Queen Leene in 600 AD alters the timeline, it becomes part of the castle along with a judicial system . The series of towers connect to the castle and each other by long, thin bridges.
Altering the Timeline
When Crono and crew kill Yakra and free Queen Leene and the Chancellor, the king and the Chancellor decide to create a justice system to punish criminals like Yakra. Part of this new justice system is a set of towers where the worst criminals of Guardia are kept locked away and punished.
Unfortunately, the prison towers are anything but humane. Squalid conditions, dark cells, and torture chambers are the norm. Indeed, there seems to be a few problems with how the courts are run, in that innocent people like Fritz are often charged and sentenced without sufficient proof.
The Trial
Crono is captured and put on trial for kidnapping Princess Nadia. At the trial, he is convicted (or exonerated) purely on character evidence, which is apparently only one of many flaws in this new justice system he inadvertently helped create. Seven elder jurors declare his guilt or innocence.
Regardless of the outcome, he is taken to the tower where he is held for three days, pending execution. Eventually, he manages to escape from his cell, where he witnesses first hand the horrors of the prison. He finds torture chambers and even a few inmates who have died and been left to rot in their cells. He is aided by Lucca during the escape, and fights off numerous Royal Guards, Gaolers, and Sentries while trying to find a way out of the labyrinthine prison.
The exit from the Prison Towers is guarded by the Chancellor's newest weapon, the Dragon Tank.