"Het Steen (The Stone), in Antwerpen, is a castle at the river Scheldt. The castle has been renovated and adapted numerous times during its history. Most likely the castle was originally a part of the city wall and dating back from the 13th century.
In 1520 the castle was renovated and extended by architects Keldermans and De Waghemakere. For a long time (1549-1823) the castle was used as a prison.
In 1862 it became an archeological museum. When the castle was renovated between 1889 and 1890, they also added a wing in neo-gothic style. Since 1952 ‘t Steen is the location for the Maritime Museum." - in: http://www.vrantwerp.com/category/city-center/het-steen/
And finally the card sent by Janett "janiD" from Germany.
"Burghausen Castle is located in Bavaria in southern Germany, east of Munich and is situated on the west bank of the Salzach River, built atop a ridge. The castle stretches for over a full kilometre and is divided into six courtyards. It is also the longest castle in Germany and Europe.
Today the castle is home to two museums and a youth hostel." - in: http://www.guide-to-castles-of-europe.com/burghausen-castle.html