This is the door to the 'Chapel in the Woods' at Cothele. Built in 1483 by Sir Richard Edgcumbe, as thanks for evading capture. From a plaque inside the chapel:

"Sir Ric Edgcumbe was driven to hide himself in those his thick Woods, which overlook the river whattime being suspected of favouring the Earle of Richmonds Party against King R the 3.
Hee was hotly pursued and narrowly searched for Which extremity taught him a sudden Policy to put a stone in his Cap & tumble the same into the Water, while these Rangeres were fast at his ffeels, who looking downe after the Noyse, and seeing his Cap swimming thereon, supposed that he had desparately drowned himself, gave over their farther hunting, and left him liberty to Shift away, and ship over into Brittaine.
For a grateful remembrance of which Delivery hee afterwards builded in the place of his Lurking This CHAPPEL"

I doubt that this is the original door!