(Part 1 of 2)

First of all, I'm glad you've read Musui's story! One of my favourite books and so useful in up-ending popular notions of what samurai were like. I remember my grand notions about what "retainer to the shogun" meant were shattered (entertainingly) when I first read Katsu Kokichi's narrative some five or six years ago.

Building a cage on the premises to imprison a wayward family member was indeed common in the Edo Period. It was a recognized legal punishment. The family was supposed to ask the authorities for permission to imprison their misbehaving member (or mentally ill member). This "voluntary" action on the family's part would show they were handling their disturbed affairs on their own, that the disturbance wouldn't spill any further into society, and the authorities didn't need to punish the family. This was not an individualistic society; one family member's actions reflected on the entire family. In this case, if Katsu Kokichi's father and elder brothers had let him continue his vagabond ways, both the Katsu family (of which Kokichi was the adopted head) and his birth family could have been censored, had incomes reduced or at the worst, been disbanded.

Katsu's three-year stint in the sitting room cage was not a terrible one. He boasts he could get the bars loose after two months, but that isn't a great achievement. A hint to the nature of his cage can be found in another account of cage-building, an 1858 letter quoted in Yamakawa Kikue's Women of the Mito Domain. (p. 186)

A flimsy structure may do when it's only a case of making a gesture towards society, but in the present situation, one couldn't rest until the cage is secure.

I'll return to why Yamakawa's ancestors wanted a more secure cage, but it seems clear that Katsu Kokichi's first cage was "a gesture to society." He doesn't elaborate much on what he did after he got the bars loose but that suggests he didn't even stay in it all the time. His father had presented the cage to him as a means of forcing him to settle down and focus on his studies. He mentions that he could have visitors in the cage. He doesn't mention that his son and heir Rintaro, the future Katsu Kaishu, was conceived in those cage years, so we can see he wasn't without his wife's company. His stint in the cage seems mostly like house arrest, even though there was a physical structure.

You'll remember there's a second cage in Kokichi's life: a more formidable structure his brother Hikoshiro had built in his garden. While Kokichi is busy living the gangster life in Yoshiwara, his brother takes steps to save the family. Kokichi describes the cage as follows. (p. 106)

I went out to the garden. The cage was sturdily built with double enclosures.

This wasn't the social gesture of his wayward youth. This was a serious prison meant to keep him imprisoned and punished. Kokichi boasts how he weasled his way out of being locked up by playing on his family's emotions, but since his solution was to apologize abjectly to his brother, quit his Yoshiwara work, and soon after resign the headship of the Katsu family to his teenage son, I rather think Hikoshiro got exactly what he wanted out of building the garden cage.

I haven't found any extant Edo Period house cages, since there's really no reason people would preserve them. It's possible there exist some, but if you google 座敷牢 ('zashikiro' lit. room prison), the image results are often of movie sets or more regular jail cells. There's a little bit more information on these house cages in the 20th century, when they were mostly used for mentally ill people. The text is in Japanese, but the Japanese Society of Pychiatry and Neurology has a page with photos of some of these early to mid 20th century cages As you can see from the accompanying house floorplans, some of these cages were separate structures in gardens, others were indeed built into the house. (I wouldn't vouch for its accuracy, but these models from the Saigo Nanshu Memorial Museum in southern Kyushu show off at least the popularized form of the Edo Period room cage.)