Castel Sant'Angelo is a great place to visit. The views from the terrace at the top are amazing and it's full of interesting rooms and tunnels that are well worth exploring. A ticket normally costs €5,00 however this is increased to €7,00 when the Castel hosts exhibitions or special events.
Audio guides can be purchased from the bookshop inside for €4,00 single or €6,00 double and are available in the following languages; English, Italian, French, German and Spanish.
For more information you can contact the Museo Nazionale di Castel Sant'Angelo at +39-06-3996-7600