The eerie Camp de la Transportation, where prisoners arrived for processing, was the largest prison in French Guiana. Convicts arrived exhausted after 20 days crossing the Atlantic and were eventually dispatched onwards from here. It's possible to visit half of the complex on your own, for which entry is free, though you'll need to stump up €8 if you'd like to visit the several exhibition halls with good interactive displays on the history of the camp in French.
The tourist office offers 1½-hour tours – some guides speak minimal English – of the complex that are highly recommended. One cell has Papillon's name engraved on it, but whether this was really his cell is up for debate.