Home to many civilizations for thousands of years, Turkey has hosted local and foreign tourists who travel to historic sites to bear witness to the fortified remnants of Turkey's ancient castles. The castles in Turkey, known as "hisar" or "kale" in Turkish, once belonged to medieval civilizations and the Ottoman, Byzantine and Seljuk empires. These ancient castles were erected in strategic locations, enabling empirical guards to observe the surrounding terrain with ease, situated at high elevations in major cities across the country. Ancient rulers were able to signal their armies of an approaching enemy and used these castles to protect inhabitants of the cities. Despite their unique design, these castles served a common purpose as defensive structures built to withstand military attacks. Today, those ancient castles are touristic spots offering amazing views of Turkey's historic cities and, if you're lucky, offer an incredible view of the Mediterranean Sea as well.Alanya castleLocated in Antalya, one of the oldest castles in Turkey, Alanya Castle was built in the 13th century under orders from the Seljuk Sultanate of Rum after the conquest of the city in 1220 by Alaeddin Keykubad I. The empire's specific architectural design included castle walls, a citadel – known as the inner castle – a red tower, a dockyard and an arsenal. Constructed on the ancient remains of earlier Byzantine and Roman castles, the castle was built after the conquest in three years, according to archeological findings. The castle served as the palace of the sultan and is located at an altitude of 250 meters above sea level, overlooking the Mediterranean on a rocky peninsula which is protected from three sides. The castle was later used for defensive purposes by the ancient Ottomans and dozens of villas were built inside of the castle walls during the 19th century. Today, the castle is an open-air museum. Visitors who purchase tickets can visit the seaside section of the castle while the side of the castle facing the inlet is open to the general public. Anyone who visits the touristy city of Antalya should visit the Alanya Castle and get a taste of Turkey's ancient history.Mardin castleAnother of Turkey's most renowned ancient castles is Mardin Castle, also known as the "Eagle Castle." Located in the southeastern city of Mardin, this castle is situated on top of a rocky hill and has hosted many emperors and the kings of ancient empires including Subari, Sumerian, Babylonian, Mitanni, Assyrian, Persian, Roman, Byzantine, Ummayad, Abbasid, Hamdani, Seljuk, Artuqid, Qaraqoyunlu, Akkoyunlu and Safavid rulers and the Ottoman sultan. Throughout its history, the castle has seen many victories as well as defeats. A king known as Shad Buhari, who idolized the sun in 330, lived in this castle for 12 years and was cured of a terminal illness in this castle. Home to many stories of historic civilization, the castle bears witness to history. Located on the outskirts of Mardin, archaeological findings here indicate that the castle was used for residential purposes as well. According to famous Ottoman researcher and the author of many books, Evliya Çelebi, who has studied the Ottoman Empire for 40 years, the castle also served as a storage facility of ammunition and food kept in rooms inside of the castle. What makes the castle so unique is that it has never been conquered – even though it hosted many civilizations throughout history. Many poets have written about Mardin Castle, telling of its heavy, powerful doors and magnificent views. The structure measures 1 kilometer in length and between 30-and-150 meters in width according to various measuring points in the castle. Sitting 1,000 meters above sea level, this 3,000-year-old fortress was of vital strategic importance. The castle was closed to tourism during the Cold War period and used by NATO and has also been used by the Weather Radar Commandership for the past 60 years. Many civilian attempts to restore the castle have been met with bureaucratic obstacles. After a long wait of 218 years, restoration work on the magnificent castle finally began in March. The restoration costs approximately TL 20 million ($9 million) and the castle is expected to open to tourists soon. If you have a chance to visit Mardin, which is one of the most multicultural and historic cities in Turkey, seeing Mardin Castle is one to be on top of your to-do list.Rumeli castleAccepted as a legacy of the Ottomans, Rumeli Hisarı, a castle located in the Sarıyer district on Istanbul's European side was built across from the Anatolian castle, Anadoluhisarı, on the Asian side of Istanbul between 1451 and 1452 upon the order of Ottoman Sultan Mehmet II, one year before the conquest of Constantinople by the Ottomans to prevent the Crusaders from coming from the Black Sea. The location of these two castles is known as the closest point of the Bosporus. According to legend, the castle was built in three months and is located on an area measuring 31,250 square meters and has a magnificent view from the castle. After being partially ruined in an earthquake in 1509, the castle was renovated in a short time only to be damaged again in 1746 during a fire which damaged the castle' wooden structures. It was later renovated by Sultan Selim III between 1789 and 1807. Today, the castle serves as a museum and open air theater. Some remains from the Ottoman Empire are also on display here. There are dozens of fish restaurants nearby and in the summer, concerts are also held in the castle. When visiting Istanbul, Rumeli Hisarı is a must-see spot to witness the story of one of history's most famous structures.