This imposing building by the Seine is called the Conciergerie. It’s a vestige of the Palais de la Cité, first royal residence in Paris.
The building takes its name from the concierge (janitor) appointed by king Charles V to supervise the royal administration. The four big towers are actually remains of the medieval palace’s walls.
Abandoned by the court at the end of the 14th century in favor of the Louvre, the edifice then became a courthouse and some rooms were converted into cells.
During the French Revolution, the Conciergerie served as the headquarters of the revolutionary court. That’s where most of those opposed to the Republic spent their last days. Marie-Antoinette herself was held there for two months before being led to the scaffold.