This fort was constructed by Shri Chhatrapati Shivaji Maharaj, the Chhatrapati of the Maratha Empire. The main object was to counter rising influence of foreign colonizers (The English, Dutch, French and Portuguese merchants) and to curb the rise of Siddis of Janjira. The construction was done under the supervision of Hirojee Indulkar, in the year 1664.. The fort was built on a small island which was known as the Khurte bet.
Over 4000 pounds of lead were used in the casting and foundation stones were firmly laid down. Construction started on 25 November 1664. Built over a period of three years(1664 ―1667), the sea fort is spread over 48 acres, with a two-mile (3 km) long rampart, and walls that are 30 feet (9.1 m) high and 12 feet (3.7 m) thick. The massive walls were designed to serve as a deterrent to approaching enemies and to the waves and tides of the Arabian Sea. The main entrance is concealed in such a way that no one can pinpoint it from outside.
At a time when traveling by sea was banned by scriptures, this construction on an island represents the revolutionary mindset of its engineer. A remain of an iron mould can be seen.