Although the scenery all around the castle is impressive enough, the location of the building is more impressive still – and is the key to its history. The mountains in front of the castle fall directly into the lake, with only the narrowest of through-routes between the sheer rock wall and the water. Directly opposite the defile, a razor-edge, sheer-sided islet rises from the water, of which only the very top is visible. This is where Chillon sits: if you were to drain the lake, the castle would teeter above an incredible drop of over 300m, as high as the Eiffel Tower. Such depths are cold and the lake’s weather is capricious, making attack from open water extremely unlikely. Equally, the road is narrow, the heights are virtually unscaleable, and there’s no other way to pass, making it impossible to avoid the castle. Whoever controlled the castle could control the traffic, and exact tolls from a position of unassailable security.

In Bronze Age times, there was no path around the lake – travellers had to climb the steep, 200m slopes at Chillon to a village on the heights above, then drop back down to rejoin the path. The Romans cut a narrow ledge along the lakeshore, and also opened up the Grand-St-Bernard pass over the Alps further south, turning the road past the unfortified islet just offshore into the only route connecting northern and southern Europe through the mountains. By the Middle Ages, the quantity of traffic meant the road had to be widened and also that a form of toll could be set up. The village above was abandoned in favour of a new town (ville neuve, today’s Villeneuve) built on open, accessible land a little way south on the valley floor. First surviving mention of a “guardian of the stronghold of Chillun” dates from 1150.

The Counts of Savoy, particularly Pierre (1203–68), made Chillon a princely residence, also developing Villeneuve into a major trading centre which poured tolls and customs duties into Chillon’s coffers – in 1283, on average, one horse rider and perhaps a dozen foot travellers were crossing the Grand-St-Bernard pass every hour of daylight, on every day of the year. Pierre’s architects and engineers transformed Chillon, rebuilding the half facing the shore as a fortress with three strong towers and a keep, and filling the half facing the water with grand halls and royal apartments.

As the Savoyards extended their influence north to the Aare and began to threaten the Habsburgs, Chillon became their military and naval headquarters. The castle was both the centre of court life and a much-feared prison: when plague broke out in Villeneuve in 1348, the town’s Jews were accused of plotting with Christian accomplices to poison the water supply, and large numbers of both were tortured in Chillon’s dungeons before being burned alive. By this time, the Gotthard Pass further east was in use, and the transfer of traffic away from Chillon and the Grand-St-Bernard led to the castle’s terminal decline as a military fortress, although it remained handy as a secure jail. In 1530, the Savoyards imprisoned a Genevois scholar, François Bonivard, at Chillon for inciting the Genevois people to form an alliance with the Swiss against Savoy. They left him shackled to a pillar in the dungeons for six years, until his release in 1536, when the Bernese army swept down from the north, briefly bombarded the castle from above with their newfangled mobile artillery, and took control. The chief legacy of the Bernese bailiffs’ 200-year residency at Chillon was an abundance of painted bears (the symbol of Bern).

Fortunately for posterity, Chillon became a quiet backwater. In 1816, after Vaud had won independence from Bern, Byron (aged 28) and Shelley (24) visited the castle on their tour of the lake. A guide took them into the dungeons where Bonivard had been shackled and wove enough of a tale around him, and around the castle’s history, to catch the poets’ imagination. While bad weather grounded them in a hotel in Ouchy, Byron scribbled out his Prisoner of Chillon, a long narrative poem supposedly spoken by Bonivard (but entirely fictitious throughout), which celebrates the cause of individual liberty, and which brought Chillon to the attention of the wealthy tourists who were starting to explore the Alps. Archeologists and historians launched renovations of the crumbling infrastructure in the late nineteenth century, which restored a great deal of the castle’s original grandeur. Work to maintain the castle continues today.