Khanderi is a small island fort in the Arabian Sea, a short boat trip off the Konkan coast south of Mumbai. Shivaji built it in the late seventeenth century to challenge the Siddis, the Abyssinian-descended admirals of Janjira, who held the neighbouring island of Underi, and the two forts glared at each other across a narrow stretch of water for decades. The Marathas built Khanderi first, and the Siddis answered by fortifying Underi, and the sea between them saw skirmishes and cannon duels. Today Khanderi carries a lighthouse and the remains of walls and bastions, and its rocks are surrounded by fishing boats.
From the mainland you cannot ride to the fort; you need a boat, and the crossing is dependent on tides and weather. Boats do not run in the monsoon, so the visiting season is from October to May. The nearest mainland town in the data is Alibag, about nine kilometres away, a coastal town with a fort of its own, Kolaba, which stands on a rock in the sea and can be reached on foot at low tide.
The ride along this coast is a different thing from Mumbai's chaos. Roads run among coconut and betel palms, past salt pans and small fishing villages, and the sea appears and disappears between the trees. The road quality varies, with some good stretches of asphalt and others broken by monsoon damage, and traffic is mixed with buses, trucks and local scooters. The light is strong and the humidity high, especially before the rains. Watch for sand on the road near beaches and for stray dogs and cattle on the highway.
Alibag has fuel, shops, small hotels and plenty of food, with fish thalis and prawn curry the local pride. Many weekenders come down from Mumbai by ferry from the Gateway of India, which makes the beaches busy on weekends. If you can, ride in the week, in the early morning, and pack a spare layer for the sea breeze. Evening light on the fort walls from the shore is a good reason to linger.