it's chinese history (but japanese and chinese history are closely linked so you see similarities). she's wearing qixiong ruqun and for those who can't read kanji and can't distinguish that kanji that are usually uncommon in jp names are being used like in this work here, (for future reference because there are too many styles of hanfu) the romaji at least of weird names that dont seem japanese like "gyokuyou" "kouken" are another tell though most works with kouykuu in the title tend to be set in china.
this is common confusion is why scanlators should go with chinese names for jp manga set in china, but then again jp styled readings are more easy to remember for manga readers, but readers can get confused on the setting.
for consistency as everything else is in the jp reading, on page 5 and 19 the clan name should be Kasen.