A List of Northern Irish Castles. Every famous castle from Northern Ireland 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 Originally made of earth and wood, later castles were built of stone, so many still survive today.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.