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 Inadiye (Fortezza) 
Fortezza is a suburb of Heraklion, built on a high hill next to the hospital of Venizelion. This hill was ideal for establishing a fortress, as evidenced by its name.
The village Charakas took its name after a large rock (charakas in Cretan dialect) on which we still see the ruins of the small Venetian fort and the church of the Transfiguration, which was restored recently. The fort consists of three rooms in which the floor is carved into the rock and covered with plaster.
The fort Malvicino or Malvesin is located on a hill near the village Keramoutsi at a position that now is named Castel Malevizi. It occupied an excellent position in the inner land, which offered great views over almost the entire province of Malevizi (getting its name after the castle).
The fort of Intzedin is the only fort on Crete built by the Turks, located on the hill Kalami, 15km east of Chania and has a panoramic view to Souda Gulf. The fortress of Intzedin was built in 1872 by Reouf Pasha, on the same location where in 1646 the first Turks built a tower, chasing away the Venetians. It was the main defense construction of the port and was named "Intzedin" to honor the first born son of the Sultan Abdul Aziz Intzedin.
The fortress of Agios Nikolaos of Kyriakosellia is located in the homonym settlement of Kyriakosellia, belonging to the province of Apokoronas. It was a Byzantine fortress built by the feudal lord of the region to secure his privileges.
Kastelli in Kissamos, which in 1966 was renamed to Kissamos, is a small picturesque town built in the gulf of Kissamos or Myrtilos, between the peninsulas of Gramvousa and Spatha. The name derives from the Venetian fortress that was built there, remains of which still exist.
The fort Kales was built by the Venetians in the 13th century AD, to protect the town of Ierapetra from its enemies. In 1508 it was destroyed by an earthquake and the Turkish raids. The damage was not repaired, perhaps because it was very serious and there was no funding.
Castle Kazarma (Casa di Arma) is built on a hill near the port of Sitia, reminding of the old times that it protected the town. The fort is the only surviving part of the old town walls, which were destroyed by the Venetians.
