This is one of two medusa heads serving as the base of one of the columns. Contrary to what people believe, this block of stone was brought here from a collapsed building from the late Roman period. The reason why it is rotated is to negate the gaze of the medusa.
The myth says that if you happen upon her gaze, you will instantly turn into stone. Certainly not something people would wish for. Surrounding the column is some water and quite a lot of coins from people making some wishes.
