My Students and I went to Sadras Fort and Alambara Fort ruins around ECR (East Coast Road) between Chennai and Pondicherry.
The matter below is picked up from some website some months ago and I forgot to keep the link. I am sorry but then how does one pick up information? From somebody else, right? Except the first person who wrote about something first time. We just need to give the URL.. and if anybody gives it to me I will post it here.
The ruins of Alamparai Fort lie near Kadapakkam, a village 50 km from Mamallapuram on the land overlooking the sea. It was constructed by musilm rulers during 1746 AD by Nawab of Carnatic Dost.Alikhan during the Mughal era.
The Alamparai Fort once had a 100-metre long dockyard stretching into the sea, from which zari cloth, salt, and ghee were exported. In around 1760, the British destroyed a major portion of the fort and more recently the structure was damaged in the 2004 Indian Ocean earthquake. Presently it is a picturesque and serene spot with backwaters and cool sand. Local fishermen assist tourists in a pleasurable boating experience around the fort. The fort can be reached on driving down the East Coast Road, about 100 kilometers from Chennai, 50 kilometers from Pondicherry.
However, by 1760, this fort was destroyed by the Britishers. This fort was also destroyed by 2004 tsunami. Alamparai fort is one of the major spot controlled by State Archaeological Department. On the east side of the fort is a marvelous view of the backwaters. The waters lap gently against the edges of the fort, and you can see the sea in the distance. The hypnotic blue of the sky and the sea are soothing. A boat ride along the backwaters is another exciting possibility. Some recent history - parts of the Vikram-and-Surya-starrer "Pithamagan" were shot at the fort, says a chatty local. Fresh fish from the fisherfolks' evening catch is on sale in the village of Kadapakkam.
Sadras Fort [I have the link to this matter copied below
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http://www.parentree.in/sathya/journal-203/Sadras-Fort---Day-trip---Chennai.html
Sadras (Saduranga pattinam) is an old Dutch fort that is located just past Mahabalipuram, near the Kalpakkam atomic power plant.
It is next to the beach and is a nice place to have a small picnic with children. The fort is not large (only about 100m wide and 100m long) and has only a couple of small rooms inside it. It has thick walls though. At the gate, there are couple of old cannons and inside the fort is an interesting Dutch cemetery and a gallows.
There is a granary building on the far right corner of the fort. You can climb to the roof of the granary and have a small picnic while seeing the waves crash on the beach. Children can learn about history and old forts also. The place is reasonably well maintained and is not crowded at all. If the gate is locked, just cal out and the caretaker will open the gate for you.
How to get there?
Drive on the East Coast Road. Go past Mahabalipuram and then just as you see Kalpakkam power plant (which is far to your left), there is a left turn to go towards Kalpakkam township. Follow that road for a couple of miles and it will lead you straight to the fort. You will go past the Kalpakkam atomic plant.
It is at most 5 to 10 minutes, after you cross Mahabalipuram. So it is an easy drive from Chennai and will not take more than a few hours.