From Tatters of the King
Yhtill is one of three cities in the land of Hastur (though this name wasn't mentioned in the play as performed), along with Alar and Carcosa. Yhtill sits on the lake of Hali. It has a hereditary monarchy descended from its first king Aldones (Yhtillic for 'father'). The current ruler of Yhtill at the time of the play Carcosa, or The Queen and the Stranger is queen Cassilda.
Yhtill is at war with Alar and has been for a long time. The City has become stagnant, almost detached from time.
Yhtill appears to have some odd views on incest as the succession of the rulership of the city (symbolized by the possession of a diadem) is decided by the queen's daughter choosing one or her brothers to marry.
The rulers of Yhtill fear receiving the Yellow Sign.
The city (and its rulers) are associated with the Hyades.