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.ome true castles were built in the Americas by the Spanish and French colonies. The first stage of Spanish fort construction has been termed the castle period, which lasted from 1492 until the end of the 16th century.