On 28th November 1670, Shivaji visited the iseland with 3000 contingent. Troops surveyed the iseland for three days. This alerted British garrison in Mumbai as, allowing Shivaji to construct fort so close to Mumbai was the last thing they would wanted. After 8 years, on 14th Aug 1678, the construction of fort started. The fort was extremely important as it was placed between Janjira of Siddi & British of Mumbai & would forbid reinforcement to Siddi from British.
British men tried hard to prevent this act & sent message to Maaynak Bhandari, who was the officer in charge of Khanderi & asked him to stop construction as British claimed that the iseland was given to them by Portugees along with Mumbai. Maaynak wrote in reply "I obey orders of my master & cant stop construction till my master asks me to do so". British took agressive stance on this reply & deputed warship "Hunter" along with 3 other ships near Khanderi. In reply Shivaji garrisonned Khanderi with 150 troops & 4 cannons.
Daulat Khan, another naval officer of Shivaji provided huge re-inforcement to Maaynak on 11th Sept 1679. British promptly sent another warship, "Revenge" to help existing fleet & then started the first big naval battle between Maratha & british navy. British fleet was badly defeated & had to retreat. This clash was followed by many small clashes between the two.
On 17th Nov. 1679, Maratha captured British warship named "Dove". This was a big setback for so called "Royal navy" & as a result they called off the war. Fort remained with Shivaji still he was alive but after his death, Siddi immediately conquered the fort taking advantage of instability.
In 1755, Khanderi was conquered by Marathas & then ruled by Angre family till 1818.
There is a big lighthouse in the fort which is interesting. This is to show rocky patches in the sea. This was constructed in 1867.
One can see cannons mounted on carts inside the fort
How to Reach
One can travel to Thaal, which is southwards of Mumbai in Raigad District. ST buses fly frequently between Mumbai & Thaal. There are two iselands in the sea, which are normally visible from coast of Thaal. The one near to the sea shore is Underi & the one in deep sea is Khanderi.
Entry into Khanderi fort is restricted & one needs to take permission from Archiology department. From Thal, there is no specific ferry service but one has to find a fisherman who could take you to the fort.