Tournelle? An old square tower from the medieval wall put up by king Philippe Auguste in 1185... which stood on the corner of the current bridge, on the quai de la Tournelle.
This tower used to defend the river Seine with another tower located in the Saint-Louis island (the Loriot tower), says the book Dictionnaire historique de la ville Paris et de ses environs (Pierre Hurtaut). Huge chains were tied to those towers, crossing the river to prevent boats from passing by.
In 1632, a royal edict said that all the persons sent to the galleys were locked here. Prisoners waiting for the departure for Toulon, Rochefort or Brest prisons: a trip which happened twice a year, in May and in September, says the book Connaissance du Vieux-Paris by J. Hillairet.
Before, the galley slaves used to rot in the Conciergerie jail, in Paris, in disgusting conditions... But saint François de Paule, who witnessed their inhuman daily life, interceded with the king.
So, detention conditions were better than in the Tournelle jail. But it wasn’t fantastic! The book Les galeries du Palais de justice de Paris (part 4, Amédée de Bast) says that during the floods, prisoners were up to their waist in water, in gaols. It was:
"a cesspool, a Hellmouth where spread shouts, horrible swearwords and curses, day and night. We heard chains jangling, because the prisoners, confined in narrow cells, had irons on hands and feet. But we also heard ferocious roars and obscene songs.
Criminals feared that jail. Citizens hated it. This Tournelle jail, sitting near the river banks like a toothless and evil fairy, kept in her stony bosom all the worst immoral instincts of Paris’ rabble."
The jail was closed in 1790 and the Tournelle tower was demolished.