I hadn't really understood that piracy on the high seas was a sport of kings. If you had a boat with guns, you could be a member of your king's team, and go raiding the opposing teams' ships. Score was kept in terms of ships, personnel and treasure taken. If you were a legitimate team member you got a proper certificate with stamps and signatures and everything, so that if you were taken by an opposing team you were held as a prisoner of war. The rules were that as a team member, when you won, you had to turn over your spoils to your king, who then gave you a reward. But if you raided on your own account, with no certificate, you were classed as a pirate, in which case defeat resulted in death by hanging.
There are three towers in La Rochelle that you can visit. We got a three-in-one deal ticket, valid for long enough, and spread our visits over the duration of our stay. This tower is the lantern tower, so called because it was originally a lighthouse before it became converted into a prison. It held captured pirating team members for a few weeks or months before an exchange deal between kings sent them home again (for legitimate raiders) or until they were hanged (pirates).