A castle (from Latin: castellum) is a type of fortified structure built in Europe and the Middle East during the Middle Ages by nobility.Scholars debate the scope of the word castle, but usually consider it to be the private fortified residence of a lord or noble. keep was a great tower and usually the most strongly defended point of a castle before the introduction of concentric defence. Keep was not a term used in the medieval period – the term was applied from the 16th century onwards – instead  donjon  was used to refer to great towers, or turris in Latin.