In front of  the Knights Castle’s main entrance is the enormous plane tree with a perimeter of  10 metres.
This is the famous ‘plane tree of Hippocrates’ supposedly planted some 2400 years ago by the great physician himself. According to the local traditions, it was here, under the tree, that Hippocrates taught many of his pupils. Another legend tells us that the ancient Koans set up a statue of their famous fellow islander Philetas (334 – 270 BC), a poet, beneath the tree.
Since the plane tree stands close to the harbour in an area that has been cool and shady since ancient times, much of the official life of Kos town has taken place here throughout many periods of its history. It may well be on this spot that St. Paul spoke to the inhabitants of Kos and spread the word of Christianity, the new religion.
It was only natural that this aged and imposing tree should have become associated in the local mind with various myths and legends. It was seen as a tree of great power, and various customs, many of them still honoured on 1 September each year.