Hyacinth Town is known as the town of the Mystics/Youma of Ceres, though it's an interesting little place in that there's actually two locations in the same general spot. The first location is Hyacinth Town itself, which is well known as a medical research area, as almost all of the buildings above ground are home to scientists or doctors that study cells, diseases, illnesses, poisons and so on to try and learn more about them (although not necessarily to learn how to cure diseases). Above ground, there's an odd blob-like object that, when activated by one with Mystic blood within them, blooms into an odd flower one can step into the centre of in order to be taken to the underground castle grounds.
Hyacinth Castle is quite a lot busier than the town above it, full of regular citizens, royals and Familiars everywhere. It's here where the produce and goods are sold, such as musical instruments or unique foods that are grown underground here (as the land above them isn't fertile and can't grow anything more than the hyacinths that the place is known for). The Castle itself is fairly large, if not strange inside, resembling a giant hyacinth and gaudily decorated inside. It does have an interesting spiralling staircase that would lead to where Mother Mystic and her knights reside. If one is too small or too scared to walk up the (barless) staircase, however, there's also a pulley system that will allow one to simply pull themselves up. If one were to fall, though, it's likely one of the Ciel Mystics there or a flying Familiar would help that person out.
As can be expected, the town is hard to find, and the castle is harder to access still. Until the relationship between Youkai and Youma improved, there were no Youkai around the area since they couldn't naturally access it. Now, one may occasionally see the odd Youkai hanging around, bringing herbs and rarer materials to their Youma friends to help them out.
The town has a reputation for being poisonous in some ways, unfortunately, due to the kinds of people that have originated from it.