It's a Gupta empire outpost fortress in North Bengal, to prevent enemy attacks from China, Tibet. Once upon a time, the Nal kings ruled here. There is a myth, that the fortress destroyed heavily by an earthquake and sunk into the mud.
Guards of the Fort decided to stay on ramparts rather leaving the ruins. They all get sunk in the mud with the Fort. Only a small portion of the upper structure is still visible on surface. Few incredible trees are visible near the ruin, locals call them Ramgua, they bleed like animals when you nail them with a sharp instrument. Locals say that the guards buried alive here, were turned into trees.
Forests took over the ruin, during the Bengal Sultanate one Rajbangshi chief took shelter here, his name was Chila Roy. The name Chilapata probably derived from his name.
Now, this is a prominent elephant corridor between Buxa Tiger Reserve and Jaldapara National Park, located only 20km from Alipurduar. One can visit the ruins inside park, and watch wildlife like elephant, deer, Gaur, leopard from a watch tower. The area is populated with semi civilised Rava people.
While travelling you should take forest guard escorts with you, because you may find wood poachers with sharp weapons inside. The ruin in the forest is heavily populated with poisonous snakes and nothing is there beyond the gate type structure in image. Archaeological researchers have found various gold and silver artefacts nearby.