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 
Agios Stefanos fort 
The village of Agios Stefanos (or Gra) in Sitia province, located between Pefki and Stavrohori villages, is built on a high hill. Atop the hill there is a rock which the locals call Kastellos or Fortezza, atop of which there are the ruins of a Venetian fortress.
Paleokastro fort was built by the Venetians on the site of the ancient town Aptera, which was aimed to protect the Souda Bay from possible attacks or invasions. The walls, which were oriented from north to south in some places were 1.5m wide, 3m high and was equipped with arched tanks. A wide road connected Palekastro to the sea.
Fortezza fort dominates the hill Palekastro beside the old town of Rethymnon and is one of the biggest fortresses of the Venetian Era. It has been built on the site of the citadel of ancient Rithimna and the Temple of Artemis Rokkea. The grand pentagonal fort was built in the 1573 and has perimeter 1300m long.
Small, well-preserved, leaning on the edge of a desert plain of the Libyan Sea in Sfakia around the mountains, Fragokastelo unfolds even today the skein of history, a story full of memories and mysteries. The "castle of Drosoulites" was built by the Venetians in the period 1371-1374, aiming to protect them from Sfakians, who were led by the six brothers Patsis who constantly bothered occupiers, who could not complete the project.
Above the seaside settlement of Keratokambos rises the steep hill of Kerato, also known as Kastri or Vigla, on which the ancient city of Kerea is believed to have been built. Vigla also hosts the impressive cave Nychteridospilios (meaning Bat cave), which was used as a sanctuary during the antiquity.
The fortress of Rokka is built on the steep hill of Trouli above the village of Rokka in the province of Kissamos. It is a fortress of the Byzantine period that was probably built after the liberation of Crete by Nicephorus Phocas in 961 at the site of a Roman fort.
In the Mylopotamos province, one of the most mountainous in Crete, there were many fortresses. Among them are the Castle of Mylopotamos located in the village of Panormos and Panokastri near Agios Mamas. Panokastri or Pano Kasteli is situated in a rocky position above the village Kastri. This belonged to the family of the noble Alexios Kallergis from 1341.
Spinalonga Islet 
Spinalonga is a small island located at the entrance of Elounda lagoon and north of the Gulf of Mirabello. It has an area of 85 acres and its maximum height is 53 meters. The history of the island still inspires awe. It has been a Venetian fortress, a castle colony, a rebel refuge, a place of exile for lepers, a communication link with Cairo during the Second World War.
