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Hear The Medieval Alarm That Protected The Shogun From Potential Assassins

Picture this: the year is 1601, and you are the great Tokugawa Ieyasu, founder of the Tokugawa Shogunate. You rule over a Japan that's unified for the first time in centuries. But a rise to power is always accompanied by an influx of enemies. That's why, when Tokugawa Ieyasu built his great palace Nijō-jō in Kyoto, he made sure to install the era's state-of-the-art security system: a floor that announces intruders as well as any modern alarm.

How To Turn Ninjas Into Nightingales

If your great fear is a ninja attack — and the Tokugawas had reason to fear a ninja attack — then you've got to take away their most reliable weapon: silence. Enter the nightingale floor. Every step across this floor, no matter how careful, is guaranteed to spark a cacophony of chirps and whistles. That's because even the slightest pressure causes the metal flooring nails spread throughout the surface to squeal against their metal jackets. The sound is uncannily similar to birdsong. Nijō-jō isn't the only medieval building to sport these floors. In fact, several temples in Kyoto share the feature, though their chief concern was probably thieves, not killers.

Now, you might be thinking that there are a lot more people that must walk across the floors of a lord's castle or a Buddhist temple than just wannabe assassins and thieves, right? Of course there were, and of course, they set off the alarm as well. But the priests, patrolling guards, and other residents were trained to walk only in a certain pattern, causing one particular rhythm and cadence to the birdsong. If you hear an unfamiliar song, it means somebody's there who's not supposed to be. It's all a pretty ingenious system — a floor-wide pressure-sensitive alarm invented several centuries before electronics would make such a thing possible in the Western world.

Hear It For Yourself: The Nightingale Floor At Erin-Ji Temple

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The Risks Of Lordship

Why was Tokugawa Ieyasu so afraid of this kind of attack? Well, one of the era's largest ninja strongholds was located only 65 kilometers (40 miles) from Nijō-jō. This all might sound a bit fantastical, but that's only because of our skewed perception of ninjas. In fact, ninjas were more likely to dress as simple farmers in order to blend in and get information — and most of them probably just had to look in the mirror to find a farmer costume.

Rather than relentless, deadly assassins, ninjas were commoners who were hired by some lords to spy on others (or who might do some spying for their own benefit). That ninja stronghold outside of Kyoto was more of a community outside of the feudal system; one that defended itself with subterfuge instead of expensive soldiers. The group may have sold its services to the highest bidder, or simply used them to keep itself safe from the Shogunate.

In any case, the next time your floors squeak, imagine yourself as the lord of a sweet anti-ninja palace. It's not a bug, it's a feature.

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Written By Reuben Westmaas July 20, 2017