A List of Finnish Castles. Every famous castle from Finland has been included. Originally made of earth and wood, later castles were built of stone, so many still survive today. Spanning a period of 900 years during the middle ages, castles come in many styles of architecture and design. Despite their differences though, they all have one feature in common-- they are fortified. Castles are defensive structures specifically built to withstand attack in the medieval world. Technologies such as arrowslits, murder-holes, and moats further improved their capabilities. Castles finally fell out of favor with the advent of gun powder, as offensive technologies, such as canons, rendered castles largely obsolete.