Exomvourgo or Xobourgo is a steep granite mountain situated in central Tinos. It forms an imposing landmark with its rugged, craggy form, overlooking most of the island. Its obvious use as a natural fortress was quickly realized and Xobourgo has been used successively as a stronghold throughout history. The old fortification consists of a Cyclopean masonry wall of which a small part still remains.

During the Venetian era, when pirate invasions became common, the capital of Tinos was moved inside a fortified castle built on top of the mountain. The castle was named St. Helen after the temple built on the peak. Today Xobourgo is a popular touristic attraction both because of its unusual appearance as well as the endless view it offers. It is accessible through designated footpaths beginning from several villages built on its slopes.