On the rocky cliff where the Cave of S. Giovanni d’Antro is located the visitors can admire the remains of fortified artefacts, some of which can be found already along the track. There is evidence that the cave was occupied by men since prehistory, and – for sure – during the ancient Roman times, when it is presumed the existence of a fortified placement linked to the overall controlling system of the Eastern boundaries of the X Regio end of the Venetia et Histria.

During the Early Middle Ages the cave became also a worship location: here in fact is kept the gravestone of the deacon Felice, who seems to be – according to some indications – the deacon who received in 889 from Berengario I several properties in these lands. Perhaps at those times a “sacellum” (a small shrine) dedicated to Saint John the Baptist and to Saint John the Evangelist was already erected here. The appellation is typically pertaining to the Longobard culture.
During the Middle Ages here was settled a patriarchal Gastaldate (1034-1077) and, following, a

Feud of the Lords of S. Giovanni d’Antro, as indicated in certain Late Middle Ages documents.

In 1383 the patriarch endowed a certain Tomaso Formentin with the castle of San Giovanni d’Antro, deemed to be absolutely unassailable in force of the particularly protected position. The fortified cave could shelter many people, moreover the only overhead entrance, a postern type access, could become quickly inaccessible by removing inwards the wooden ladder.  In the cave are also kept the remains of a grindstone used to mill wheat and an oven, perhaps used when the castle was inhabited.

Long the narrow track to the cave it is possible to trace a medieval wall, approximately two meters high, some protruding stones with a hole, were used as stands to hold torches. Some remains of walls close to the entrance very likely belong to a ruined tower.