Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. List of castles of Poland. A castle is a type of fortified structure built in Europe and the Middle East during the Middle Ages. Scholars debate the scope of the word castle, but usually consider it to be the private fortified residence of a lord or noble. This is distinct from a fortress, which was not a home, and from a fortified town, which was a public defence, though there are many similarities between these types of construction.