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 
Liopetro Fort 
The fort of Liopetro was located in Fatsi position, near the village Hamezi (Sitia province). It is believed to have been built on the site of an older fort, of which the tank survives. Liopetro is built on a steep hill with panoramic views to all sides. Atop of it, there is the scenic chapel of Prophet Elias and the remains of walls, tanks and other buildings.
The Fort of Kavalos is located at an elevated position east of the village Larani and apparently belonged to a lord of the area who wanted to control his lands.
Close to the scenic village of St. Thomas (Agios Thomas) in Heraklion there are the ruins of one of the largest fortification works in the Cretan inland. Atop of a high hill overlooking the valleys of the Prophet Elijah and Venerato, you will find the ruins of Melissa fortress build during the 2nd Byzantine Era (11th- 12th century).
In the province of Mylopotamos, one more mountainous provinces in Crete, there were two fortresses. The first was built in the seaside village of Panormos, while the second near th village Saint Mamas.
The fort Castel Pediada was not built on a hill, but in the valley of the current town of Kastelli Pediados. It was mainly an administrative building, since it was housed several administrative and military authorities.
The Mirabello province took its name after the homonym fort, located in the current site of Agios Nikolaos. There were more fortresses around, with the most important being that of Spinalonga, which protected the salt mines of Elounda, and some small castles in Kritsa, in Oxa mountain and, perhaps, in Kastelli of Fourni.
A steep cliff rises above the village of Malathyros, on which the Byzantines built a fortress to control the province of Kissamos. Ascent is possible from Malathyros. The fortress had a double fortification wall reaching a wall width of 2.30 meters and 12 semicircular fortification towers, while traces of cisterns and other buildings survive.
16km south of Rethymno, near the settlement of Monopari in Varsamonero, there is a steep and isolated hill, encircled by the local torrent. The hill is called Kastellos and, atop of it, the Genoese pirate Henry Pescatore founded one of the 15 forts built by the Genoese all over Crete in 1206. The fort was named Bonriparo (meaning strong fort), after which the village Monopari is named.
