Marshalsea was a special kind of jail that used to exist in England a long time ago. When grown-ups did things that were against the law, they would be punished by being sent to Marshalsea prison. This was a really tough place to be because people who were sent there were often treated very badly.
Imagine if you got in trouble at school for not following the rules. Instead of just getting a time-out or a scolding, you were locked in a room all by yourself and not allowed to see any of your friends or family. You couldn’t leave the room, couldn’t play with toys, and couldn’t watch TV.
That’s kind of what it was like to be in Marshalsea. Except it was even worse because people had to pay money to be released from jail. If you didn’t have enough money, you might be stuck there for a really long time, even if you didn’t do anything really bad. And if you got sick or hurt, there weren’t any doctors or nurses to help you feel better.
Thankfully, Marshalsea doesn’t exist anymore. We now have better ways to punish people who break the law that are kinder and fairer.