I'm sure you are wondering what the hell I'm talking about with the title of this post, but the concept is pretty simple... You take a period of history or a series of famous/infamous events, turn all or most of the male personages into females, and then plop your main character into their midst, evolving a story from there.
A few examples of this...
The Koihime Musou, Sengoku Koihime series by Baseson
Sengoku and Sangoku Hime series by Gesen
Kikan Bakumatsu Ibun Last Cavalier
Koihime Musou is the one most of you will probably be familiar with, and it is the basic model for most of the type. It is based in the era of the fall of the later Han dynasty in China, portrayed in the ancient story Romance of the Three Kingdoms. This is one of the two eras most favored by those making games, VNs, anime, and manga in this particular little niche, and it is also one of the easiest ones to use, as the story has made at least some headway in most of the countries of the world (it was translated into English first over a century ago) to one degree or another. In a way, Koihime Musou (the original) is a classic example of the genre, as the events portrayed generally only vaguely resemble the ones that existed in the original story, even when you set aside the issues of all the characters being female except the protagonist. Shin Koihime Musou, which follows each of the three factions in separate story arcs (rather than a single arc with Liu Bei's side unifying China like in the original) is generally better written and actually uses historical events in most of the paths, up to a point (where it inevitably diverges radically).
The Sangoku Hime series is a strategy-VN series based in the same era as Koihime Musou, with the big difference that the protagonist is both fairly irrelevant to the story and the events are fairly close to the original material, up to a point (in fact, this is the biggest draw of the series, other than the surprisingly well-done story sequences in some of the cases). This series suffers from definite 'strategy fatigue', though... as fighting your way across China tends to rapidly become monotonous, with actual plot tending to only come up at the beginning and end of each prefecture's invasion, meaning you can go five or six hours without seeing anything in the way of story except sex scenes (of which there are generally a ridiculous number...).
Chuushingura follows the story of the 47 Ronin, a famous story of Edo-era samurai that was the inspiration for most foreigners' impressions of the samurai caste. In this case, while the events in each 'arc' generally follow the basic outline of the original story, there are divergences in perspective based on where the protagonist chooses to go, and the final story diverges immensely from the original. This VN doesn't have any gameplay, and it was originally a doujin VN series that went commercial a few years ago in the form of a rewritten, partially redesigned omnibus VN.
The Sengoku Hime series by Gesen is based in the Sengoku Jidai (Japan's Warring States period). In this one, you generally take on the role of a strategist (whose actual level of influence varies from path to path) who serves with one of the major clans of Japan as they set out to conquer the country. Generally speaking, the balance of story to gameplay in this series is better than in the Sangoku series by the same company, but the pacing is still fairly shitty due to the intervention of the gameplay. Depending on which faction you choose (generally if you choose the Tokugawa or Toyotomi) you can get a fairly accurate replay of past events, with a lot of differing details. However, if you pick certain factions (the Oda, the Date, the Ashikaga or the Shimazu) you tend to get stories that radically diverge from history even if you ignore the issues with female clan lords and the like. This is inevitable, considering the sheer chaos of that era (the concept of the 'honorable' samurai as foreigners like to see it portrayed is a product of the Edo era that followed after Tokugawa's unification of Japan), with the betrayals, massacres, religious rebellions, and the like...
Koihime Sengoku (the new version of which I am playing right now) is also based in the same era, but, similar to the original Koihime Musou, diverges radically from history relatively early on. In particular, it should be noted that while certain famous events and relationships were deliberately reproduced for the pleasure of historical romantics (the Takedo-Uesugi rivalry, Oda Nobunaga's relationship with Mino's deceased leader, the Viper, etc), a lot more of it is different. This VN is significantly different from Koihime Musou in that it has a far darker, more violent atmosphere (lots of killing, and some of the heroines are... not good people, lol), and the protagonist is far more active as part of the story (in fact, he is the nephew of the protagonist from Shin Koihime Musou).
Kikan Bakumatsu is based in the highly-romanticized period of the fall of the Tokugawa Shogunate after Perry's 'black ships' opened Japan. It covers a lot of the political and ideological conflicts wracking Japan at the time fairly accurately, while focusing on the infamous Shinsengumi (who are often thought of as the last samurai). This period is also covered in the otomege series, Hakuoki, though the only common element between them beside that is the existence of a fantasy element. Depending on the path you choose in this one, you can radically alter history and the life and death of certain characters, but if you choose one of the Shinsengumi heroines, the basic path in life of the protagonist, Okita Soujirou, goes pretty much as history portrays (it is pretty sad).
Generally speaking, due to the depth that can be added to the setting for these stories, they tend to be good more often than most... and that is generally why I tend to look forward to them, lol. However, if you aren't a history buff, some of it will just go over your head, and the language in a VN based in these eras will frequently be incomprehensible for the inexperienced. To be blunt, anyone who reads one of these should have a degree of pre-existing knowledge if they want to really enjoy them... otherwise, it isn't nearly as fun to see how the writers choose to twist things out of shape, hahaha.