10km to the north of Alexandroupolis you will find the village Avantas.  Here, on Boz-tepe Hill (grey hill) 3 towers from the castle of the Gattilusi family, who had the area under their command during the 4^th Crusade, are preserved. Although it can only be accessed by foot in a relatively gnarled area, the unrivalled beautiful view will make it worth your time.