In the summer of 1816, Byron and Shelley sailed around Lake Leman (Lake Geneva).  While on their way they stopped at the Chateau of Chillon, and deep in the dungeons, below the water-line of the lake, they visited the place where François de Bonnivard was imprisoned for four years (1532-1536).  During those years, Bonnivard had worn an imprint in the stone floor from his pacing of the confined area.

Byron was so impressed with the event, he composed A Sonnet on Chillon.  But, after learning more of the facts about Bonnivard, Byron expanded on this by writing the poem  The Prisoner of Chillon.