At the back of the town hall, you can visit Het Steen.
Het Steen is a medieval prison. You can only visit this place with a guided tour. During the tour you will hear about what it is like being a medieval prisoner (I would rather not be one) and you are being told about Balthasar Gerardz, the guy who killed Willem van Oranje. He stayed in Het Steen after the murder. All instruments of torture are still there. Just like all the carvings in the walls of the prisoners.
I went there with a small group of teenagers. Looking at the building outside they were not very interested in what they would be doing that morning. After climbing all the stairs, they got enthusiastic about the prison and even asked many questions about what they saw.
So, if you are interested in mediaeval torturing, Dutch history or if you bring teenagers to Delft, visit Het Steen.
tickets at TIP (Tourist information) / Kerkstraat 3 (Markt)