Crete in ancient times was not threatened by external enemies. The first external enemies of the island were the Romans. Therefore, till then, the fortification of cities was focused on internal enemies. After the Roman Empire (67 BC-330 AD) things changed radically. Crete became a province of the Eastern Roman Empire, which later transformed to the Byzantine Empire. At the same time, several civilizations started developing around the Mediterranean Sea.
Thus, Crete, because of its natural and geopolitical position, acquired great importance and became an "apple of discord" for the civilizations of the Mediterranean. During the First Byzantine Period (330-824) things looked calm. For that reason, combined with the internal and external problems of the Empire, the island was on the fringes of the Byzantine interest. Thus, pirate raids increased and the seaside monasteries were fortified to protect themselves.
Under these circumstances, the first storm on the island was imminent. The Saracen pirates managed to easily occupy Crete in 824, that was almost defenseless. They settled for 140 years, till 961, during which they fortified the largest town, today’s Heraklion. After continuous failures to reoccupy Crete, Nikiforos Fokas managed to liberate the island from pirates in 961. Then, the Byzantines built new walls, which managed to keep invaders away for a few more centuries.
In 1206, Crete passed into the hands of the Genoese, who immediately gave particular importance to the fortification of the island. Within a few years the fortified three castles and 12 fortresses, but still the Venetians managed to occupy the island in 1209. The Venetian Era in Crete lasted for 4.5 centuries and was mainly dependant on the colossal fortification structures. The best Venetian engineers built the Great Castle of Candia (Heraklion). The Turks managed to take over Candia in 1669 after 22 years (!) of besieging and immediately started restoring the Venetian Forts and built a lot of small fortresses across the Cretan Territory, called Koules. The Turks took care of their forts till the end of their presence on the island.
Even today, visitors can get an idea of what the fortified cities of Crete were like, as the walls are still in excellent condition. The impressive castles in Chania, Rethymnon and Heraklion and the fortresses on the islands of Souda, Gramvousa, Spinalonga are but a few samples of Cretan fort architecture.
Castles 
Turkish Forts (Koules) 
Towers and Chateaus 
Fort Kenourgio Kasteli (Castel Nuovo) 
The fort was built by Pescatore in 1206, who named it Nuovo (i.e. New), probably because it was one of the fortresses he built from scratch, without another preexisting. It is believed that this is the site of the ancient city Aina. The entrance to the fort was located on the south side of the hill, where there was a steep staircase.
East of Paleokastro (Sitia Province) rises the steep hill Kastri, which separates the beaches of Kouremenos and Chiona. The hill was called Paleo Castro (Old Fort) by the Venetians, after which the current village is named.
The Venetian fortress of Paliokastro is located 14km west of Heraklion, on the west side of Heraklion Bay. Today, only a part of the walls of the triangular fort remains alive. The fort was built on a big tall rock, near which passes the highway of Heraklion - Chania.
The fortresses built by the Venetians along the south shores of Crete were very few, perhaps because of the lack of ports and bays. One of them, however, was the fort of Selino (in the site of modern Paleochora). This fortress was built around the neck of a protrusion of land into the sea, so as to support the domination of the Venetians in the area, which was inhabited by many rebels.
The fort of Sfakia (Castel di Sfacia or Sfachia) was built on Kastelli hill, on the eastern edge of Sfakia (Chora Sfakion) town and was the last fort built by the Venetians in Crete. It was built during the 15th-16th century on the site of a previous Byzantine fort. Information about the castle is very poor, but this was mentioned for the first time in documents of 1526.
The Fortress at position Tholi is located at the homonym region west of the Alagni village and dates back from the 16-17th century. The two-storey building, parts of which are still in bad shape, was protected by a wall.
The village of Kanli Castelli, today known as Prophet Elijah, is located about 18km south of Heraklion, at the start of Malades valley. The village is situated at the foot of the steep two-peaked hill Rocca, on which even today there ruins of the fort Kanli Castelli, after which Prophet Elijah took its old name.
Kastelos Hill is located 8 km west of the city of Chania and 15 km east of the village Varipetro of Province Cydonia. It is a walled citadel (acropolis) at an altitude of 250m. The hill is considered to have been inhabited since the Minoan Era, to the medieval Times.
