The cave where John is said to have received his visions became, roughly a thousand years later, the reason a fortress-like monastery was built directly above it.
A specific cave, a specific tradition
The Cave of the Apocalypse, a small grotto on Patmos, is venerated as the traditional site where John received the visions recorded in the Book of Revelation, and the site preserves specific rock features, including a formation traditionally identified as where John rested his head, that pilgrims continue to visit as physical points of connection to the text's origin.
A monastery built to protect the tradition
The Monastery of Saint John, founded in 1088 on a hill above the cave, was built with thick, fortress-like walls reflecting the genuine risk of raids in the medieval Aegean, and it developed an extensive library of manuscripts and early printed books that, alongside the cave itself, earned the island UNESCO World Heritage recognition.
As this entry's companion article covers
The broader question of John's exile to Patmos and the circumstances under which the Book of Revelation was reportedly composed there are addressed directly in this entry's companion article rather than repeated here.