Japanese castles were fortresses composed primarily of wood and stone. They evolved from the wooden stockades of earlier centuries, and came into their best-known form in the 16th century. Castles in Japan were built to guard important or strategic sites, such as ports, river crossings, or crossroads, and almost always incorporated the landscape into their defense. Though they were built to last and used more stone in their construction than most Japanese buildings, castles were still constructed primarily of wood, and many were destroyed over the years. Today, there are more than one hundred castles extant, or partially extant, in Japan; it is estimated that once there were five thousand.