While many post-towns had bansho which were required to keep watch over the highway, there were barriers (seki or sekisho, barrier station) at intervals which had special duty to inspect each passing traveler and porter.
The primary purpose of a seki was to prevent the movement of two dangerous commodities. One was the smuggling out of Edo of women who were being kept hostage under the sankin kotai system against the good behavior of their husbands. The fear was that the delicate checks and balances which maintained the peace would be disrupted if a daimyo and his immediate family were all removed from the physical control of the Tokugawa shogunate. The second dangerous commodity was arms, especially firearms, moving toward Edo. Daimyo when they traveled to Edo were always with their armed retainers for carrying swords was the right of all samurai and samurai alone, but the size of the party was strictly limited so that no daimyo, especially a tozama daimyo, could build up an armed force which could contemplate an attack on the castle of the Tokugawa. Samurai, of course, could smuggle themselves along and be secreted in Edo, but weapons were a different matter. Firearms were particularly worrisome because a small number, if successfully smuggled, could tip the military balance quite suddenly.
Seki date back nearly to the origin of the highway system. The Nara period saw the creation of three seki which were considered crucial to the security of the “Home Provinces”, those close to Nara. One of these was located at Fuwa, near the Nakasendo post-town of Sekigahara. These fell into disuse as the central authority proved too weak to enforce its will for long. By the 14th century, seki had appeared in great numbers as local lords used them to collect tolls and control traffic through their domains. There were so many that commercial development was seriously impeded.
Oda Nobunaga and Toyotomi Hideyoshi, the first two of the three unifiers of the 16th century, abolished seki because of their interference with movement, but Tokugawa Ieyasu revived them not as toll-collecting barriers but as security devices. There were about 50 throughout the Edo period, including the two main ones on the Nakasendo at Kiso Fukushima in the midst of the Kiso valley, and Usui on the outskirts of the plain before Edo.
Each guard station was an elaborate construction, in keeping with the important role it played. Gates were placed across the road which were shut after 6 o’clock; travelers who did not make the deadline had to spend the night in the open or return to the previous post-town. All travelers passed through the guard station and were subject to scrutiny by samurai officials. Any traveler or baggage which looked suspicious was taken aside for a careful inspection or questioning. Any person dressed as a male who appeared suspicious was taken into a special room for a physical examination to determine sex. A small prison was maintained and, standards of justice being somewhat more lax than today, manacles, thumb screws and other iron implements of ‘questioning’ (torture) were kept close at hand.
While smugglers could find themselves in trouble, so too could the guards. There is a story related by Philipp Franz von Siebold, a German doctor who tended the Dutch traders at Nagasaki in the 1820s about a man who was smuggling his daughter down the Tokaido. She successfully passed scrutiny at Hakone barrier in the shadow of Mt. Fuji, but was discovered by a fellow traveler shortly after. A bribe was demanded and refused, whereupon the other traveler hurried to the seki to inform about the traveler. The guard who had been fooled realized his head might be lost if his lapse were discovered, so he arranged for a boy to be substituted for the girl and when all parties were brought together for an investigation, the smuggler was able to present a boy for examination. He thereupon flew into a rage and killed the informer, solving a problem for the guard as well as himself. Everyone parted company pleased with themselves and the Tokugawa security system was breached, but only a little.