Forts in India have been constructed to protect the settlements against hostile attack and marauding wild animals. Due to successive waves of invasion, there was an ancient tradition in India to erect massive and strong forts to obstruct the invaders. The origins of some of the Indian forts are lost in the mists of time while some have been transformed or reconstructed after the invasion of the Muslims in the twelfth century.

The ancient treatises on Indian architecture and fortification can be retrieved from the Shilpa Shastras and in some of the ancient Indian literature like the Indian Puranas and Epics. This vast pool of knowledge gives a significant account of the development of the forts in India. `Pur`, a word which is mentioned repeatedly in the Vedas, means a rampart, fort or stronghold. In other places forts `with hundred walls` are mentioned.

Kautilya, in the forth century BC, wrote extensively regarding the protective fortifications. He differentiated forts as per the preferred types of site. Water and mountain forts or fortresses were considered to be the best for to defend the populous towns. Desert and forest sites were taken to be `habitations in the wild`. In cases where a fort was to be situated on an open plain, then water was introduced and the land thoroughly cleared to form a maidan or open field of fire. The Nara Durg, a fort which was protected by men, gives and account of the Aryans correlation of a fort with a town. Lists of plans were used for defensive works of the fort, normally based on a rectangle.

The work of Kautilya reflects the ancient Shastra and Vedic rules for town planning. Usually, two major axial roads were lined up with the cardinal points, crisscrossing at a central crossing. Here the Gods were placed. The house of the king lay to the north, while the outer ring behind the ramparts reflected the path around the sacred sacrificial enclosure. Afterwards, with the impact of the Bactrian invasions in the north-west, Hellenistic ideas of planning a town became influential, with a grid pattern layout around an acropolis or and isolated central citadel. Another major part of the fort complex was the temple and, after the 12th century, the mosque. Water was always the prominent concern in the fort to ensure self-sufficiency during times of siege or duress. A large number of forts possess natural or artificial tanks within their protective ramparts and some of the forts possess underground supplies of extraordinary sophistication.

After the invasion of the Muslims, construction of forts in India reflects many of the developments which took place in Europe. The central citadel was kept isolated and secure from the expansive outwork of curtain walls. Usually the citadel was sited on a naturally defensible outcrop of rock. The forts of the great Maratha Emperors of western India like the Purandhar, Raigarh and Sinhagarh have manipulated natural terrain to their strategic advantage. At Gulbarga, in Deccan, the central stronghold followed the practice of the Crusader castles of the Near East, with a robust donjon or inner redoubt at the heart of the defense.

The importance of defense in depth in the India forts had been appreciated for centuries. There are several forts possessing concentric rings of moats and walls, with advanced gateways or outworks dominating the approach. Also, significant ingenuity was deployed in manipulating the approach to thwart the advance of an enemy. The use of right-angle bends, zigzag causeways and serried ranks of fortified gateways strengthened with anti-elephant spikes offered a formidable barrier to any hostile army.

With the use of artillery in warfare, the entire nature of forts in India changed. High, thick walls were replaced by broad, low, sunken ramparts, saved by ditches to provide a minimal target for cannon fire. With the sophistication of fire arms, massive angular, battered bastions with interlocking fields of crossfire substituted the curvilinear forms of the mediaeval strongholds. With the European influence, the great fortresses of the Portuguese at Diu and Goa, or the English at Fort St George, Chennai, or Fort William, in Kolkata, were formed on European principles, but adapted to an Indian context.