Pillow talk was a powerful weapon for gathering intelligence during the Vietnam War, with bar girls among recruits passing information to Viet Cong spymasters.
Another celebrated example of successful espionage: a simple noodle shop - frequented by United States and other military officers because of its superb pho (noodle soup) - was run by a communist commander. Waiters reported overheard conversations and their boss passed the intelligence to guerrilla leaders, who would adapt their plans accordingly.
Ho Chi Minh City - formerly known as Saigon, a name that is still commonly used - teemed with spies during the Vietnam War's two decades, culminating in communist victory in 1975.
Secret agents and their handlers met in then-decrepit hotels to deliver or receive information. Rumours about spies' identities were common topics of conversation.
But times have changed dramatically. Vietnam is at peace. Tourism has become a big industry - and faded colonial hotels are recognised as architectural gems, expensively refurbished and restored to their original glory.
In this city, colonial-era hotels are a colourful but comfortably up-scale alternative to global accommodation chains. Rooms are of the same standard and similarly furnished to those at the city's other leading hotels.
Two of the oldest, the Grand Hotel Saigon (formerly the Saigon Palace) and the Hotel Majestic Saigon - both raised to five-star standard - have become so popular with foreign visitors that new wings have been built to increase room numbers. Staff say frequent visitors can tell the difference - and often prefer staying in the original sections.
Another old hotel, the Caravelle, isn't really colonial - it opened in 1959, four years after independence - but is often grouped in this category because it, too, is steeped in modern Vietnamese history.
The Caravelle, which also sports a new wing, was called the Doc Lap (Independence) from unification in 1975 until 1998, when it reverted to its original name.
The rooftop of its old wing, now called the Saigon Saigon Bar, has panoramic city views and was a sunset hang-out for war correspondents during the conflict.
Back then the building also housed several embassies as well as the offices of several television networks. In the lobby, spies mingled with guests while waiting to meet their paymasters.
Today the Caravelle bridges the gap between old and new.
Celebrity guests have included former United States President Bill Clinton, former South African President Thabo Mbeki, Britain's Princess Anne and actor Sir Michael Caine (when he was filming a remake of The Quiet American). The film was partly set across the street in the Continental - another famous and refurbished colonial hotel.
It was at the Continental that Graham Greene wrote part of the novel on which the movie was based.
During the Vietnam War (called the American War in Vietnam), US secret agents often met their Vietnamese contacts at the Continental or nearby cafes. An inner courtyard was - and still is - a well-reviewed place to dine.
The Continental is a few minutes' walk from another hotel, the Rex, where the American military held daily wartime briefings that some correspondents nicknamed "the five o'clock follies".
Vietnam's most successful spy, Pham Xuan An, was a frequent visitor to the Continental. His cover: he worked for western publications, including Time magazine, as a journalist and "fixer". But An was secretly a committed communist, and his reports were avidly read in Hanoi.
Graham Greene also stayed at the Majestic, which William Somerset Maugham - another renowned and well-travelled author - maintained was better than the Continental. The Majestic's Cyclo Cafe remains one of Saigon's most fashionable dining spots, a place to enjoy western and Vietnamese fare in time-warp surroundings while watching the world pass by.
The Cyclo's view is improved, in these more relaxed times, by the absence of the anti-grenade grilles that were a wartime adornment.
The Majestic's bar overlooks the Saigon River, where brightly-lit floating restaurants take tourists on dinner cruises. The bar used to be favoured by French-colonial agents for rendezvous with spies, who slunk in by side doors to report on pro-independence activism.
Ho Chi Minh City is Vietnam's most important commercial city and its enormous urban sprawl is a stark contrast to the picturesque capital Hanoi, which is a couple of hours' flight-time to the north.
But Ho Chi Minh City has a compact centre, anchored by Dong Khoi (meaning "total uprising") Street.
Tree-lined Dong Khoi, on which the Grand and Majestic are located, is an important retail strip. A three-minute stroll puts me at Lam Son Square, where the Caravelle and Continental sit opposite each other, with the French-colonial Municipal Theatre (also known as the Opera House) in between.
Steel-and-glass high-rises came later to Ho Chi Minh City than to many parts of Southeast Asia, and important older buildings are increasingly being refurbished and reinvented rather than torn down.
The Municipal Theatre, along with the City Hall (now the People's Committee headquarters) and the Saigon Central Post Office (designed by Gustave Eiffel, of Parisian tower fame) are among the city's most impressive colonial-era buildings. I can reach them all in a short walk from the Majestic, where I'm staying.
And when you've worked up a thirst, here's another building that might interest you. Turn right out of the Caravelle and you'll find, a few doors down, the Lion Brewery. Vast and reminiscent of a German beer hall, it's a micro-brewery that is much liked by tourists wanting a cool drink.
IF YOU GO
Staying there: Many hotels exist in all price categories, including budget and backpacker lodgings. But for those wanting to immerse themselves in Ho Chi Minh City's past, the main historic hotels are the Caravelle, the Continental, the Grand and the Majestic.
Further information: See vietnamtourism.com.
The writer was a guest of Vietnam Airlines.