30 Prisons, some of which are dreadful places in which to be, whilst others can be surprisingly pleasant, as far as somewhere can be pleasant when you are not allowed to leave it. Some are short term places of confinement, others are where you will spend the rest of your life unless you manage to escape.
Most criminals who are locked up as a punishment or to await trial are just confined in regular cellblocks or jails, but not all of them. Some are so dangerous that ordinary prisons just will not hold them. Others have secrets that cannot be allowed to be spread, or their crimes are so dreadful that they deserve a greater punishment then normal incarceration or even execution. In yet other cases the ruler had a nasty sense of humour...
There is a person who has a well-known reputation, probably involving something illegal. But this person is either very clever and crafty, or really like his or her privacy, for no one knows who this person actually is. As an added twist, perhaps this person is of an age that would noramlly be wildly inappropriate (say, a child is a much-feared bandit) or maybe not the expected gender or even race.