Arnala fort was built on Arnala Island, approximately 8 miles North of Vasai. It is surrounded on all sides by water and so is also known as “Jaldurg” or “Janjire Arnala”. The main structure is on the north side of this longish island. The fort has broad and strong high walls with three gates – the main entrance being in the north. It has a lone tower (bastion) in the south end. The entry to this is through a small square opening – it is dark inside and the undergrowth makes it difficult to explore further. However there are trees on the outside walls, which have been used by the enthusiasts to climb on the top to get a peek inside.
There are steps leading to the ramparts (walls), and one can take a walk around the entire boundary wall. The ramparts also have number of small openings in its broad walls to allow entry in the small rooms and store houses below. Some of these storehouses are used by the villages to store their paddy crop. Along with some remains of the weapons used from this fort, one comes across a canon ball, still stuck on the inner side of the west side of the wall near the temple.
The fort was under the Portuguese rule after the Portuguese signed a treaty with Bahadurshah of Gujarat . It was later captured by the Marathas during their invasion of Vasai. There is some record of renovation of this fort by Peshwa Bajirao I after its capture by the Marathas.
Many of the villagers believe that Shivaji built this fort under the leadership of Kanoji Agare. The fort came under the British control in 1803 after Bajirao II Peshwa had signed the infamous Treaty of Vasai.
To reach Arnala, there is a regular ferry service for the villagers and visitors to the fort, but since there is no jetty or quay, one has to wade through knee deep water to reach the boats from the shore and vice versa. A 10 minutes journey by a rowboat is an exciting experience.