The fortress was built four centuries ago, to function as a hidden emergency capital should Manchu forces invade Seoul. In such an event, the elite of Korea’s Joseon dynasty would escape with key staff to Namhansanseong, which was guarded by warrior monks.

As is clear from my elevated perch – the Iwijeong pavilion – this was no mere hideaway. Rather, Namhansanseong is a sprawling, walled complex of homes, temples, armouries, towers, ceremonial halls, command posts and administrative offices, once capable of accommodating 4,000 residents.

According to information available at the small visitor centre, military structures have stood on this site since the seventh century. Its current form represents the grand citadel built in 1624; original parts of that complex remain, while others have been reconstructed over generations.

Unesco notes that Namhansanseong’s 1624 design included features intended to trick Joseon foes. Across the site were 16 gates that could not be seen from outside the citadel, allowing for the discreet movement of supplies and weapons. If any still exist, they still cannot be seen – at least by this visitor.

Some 1,940 slits were carved into parapets atop the outer walls. Any force attempting to seize Namhansanseong could be repelled by weapons fired through those openings. The Joseon soldiers manning these posts had supplies stored in 125 guardhouses built into the fortifications.

Although large sections of Namhansanseong have been destroyed – mostly in the early 1900s, by Japanese occupation forces fearful of anything that could be used as a defensive position – it remains one of the most impressive fortresses in Asia.

Namhansanseong can be freely explored and its history is partly revealed in its tombstones – stone monuments standing in a now barren field that include one marking the grave of a heroic woodcutter.

As Manchu troops stormed towards Seoul in 1636, King Injo (1595-1649), the 16th monarch of the Joseon dynasty, fled to the mountains. En route to Namhansanseong, however, many of his servants abandoned him.

To the rescue came a stranger, woodcutter Seo Heun-nam, who carried the monarch all the way to the citadel. Injo was so grateful he made the woodcutter a spy and gave him a royal gown, in which Seo was buried – beneath what is now a small, cracked upright stone missing an upper corner and bearing weathered Korean script.

Elsewhere in the complex is a stately building inside which Injo supposedly received further key help, this time from an otherworldly figure.

The king lived in this structure, called Naehaengjeon, which is perched near the crest of the citadel, enclosed by walls and flanked by a courtyard.

During the invasion, Injo awoke here one morning in a panic. He said that he had been warned in a dream that the walls of Namhansanseong were being breached. According to legend, he was right; his guards were warned and they managed to repel a unit of Manchu soldiers.

In truth, Injo had limited success in his tussles with the Manchu.

After he went into hiding at Namhansanseong, the invaders took Seoul and reduced Joseon to a tributary state. They starved Injo out, and he surrendered 47 days after disappearing behind the walls of the citadel.

Exploring further, I cross beneath the towering Hannamnu Gate, an 18th-century structure supported by 10 pillars painted earthen red, with three large wooden doors at the bottom. I head up two sets of stairs and through the smaller, tile-roofed Bukhaenggak Gate, another structure with three ground-level doors.

Beyond Oehaegjeon, which was the king’s office but is now empty, further steps and gates lead back past his home, more halls and offices until I reach the Iwijeong pavilion. Flanked by trees, the petite, pretty pavilion was built two centuries ago by a Joseon dynasty successor to the unfortunate Injo.

Nowadays, Injo is not thought kindly of in Korea, his reputation tarnished by his desertion of Seoul. Other Joseon kings have tombs in prime locations such as Gangnam, in central Seoul, but Injo’s resting place is tucked away in a forest about 30km northwest of the city’s downtown area.

As I admire the colourful paintwork of the citadel’s 18th-century royal ancestral shrine, called Jwajeon – a cluster of four small buildings, each locked to protect the spirit tablets inside – I cannot help but think it would have been appropriate to bury him here.

Having commissioned the construction of Namhansanseong, this tremendous citadel is perhaps King Injo’s greatest legacy.