Apart from economic incentive the other consideration behind building these cities was defense. Therefore, most important architectural feature of these cities was defense.
These cities were surrounded by hostile local population on one hand and European rivals on the other. With the advent of gunpowder, the design of fortification completely underwent innovative changes. The system of fortification was brought from Britain. The work on Fort St. George in Madras begun in 1640. But the defense system was unplanned in the port cities. Lying between two bays it was not conducive for withstanding sustained attack.

Calcutta was the last of the port cities to be founded by the English. It had an access to a rich and fertile and well-populated hinterland and over a period of time became the largest of the three ports. Fort William was built for Calcutta in 1712. A defensive ditch was dug in 1742 to protect these settlements from invasion by bands of Martha cavalry (referred to as bargi in local language). The defense of the fort was also not supposed to be great and after the British were defeated in 1756 it was said by an observer that many old buildings had better defense than Fort William.

Building port city on coast makes economic sense but even more than that it makes sense in terms of defense. It is interesting to note that the chief danger was from the land than the sea and therefore it always had the strongest defense towards land.

The fortress was placed on the water’s edge suggesting that in the case of danger they could have escaped through sea route. Kosambi and Brush point out how after the attack and occupation by the French Madras underwent certain important spatial changes. The fort was strengthened and enlarged; the ‘Black Town’ was demolished to create an esplanade. Thereby an unobstructed defensive zone was created on the inland side. The authors develop a schematic spatial model of these port cities.