I will try to find the time to draft out something to add some background. Just throwing out some ideas for discussion to gather some feedback for a while until there is sufficient materials.
I have some alternate suggestions for the reasons of safety from Tsaesci, though. I was thinking that given the scale that I was working if, it the Tsaesci really want to deal with them, it wouldn't be THAT difficult (these guys kill dragon-gods!). Instead I propose the following adjustments:
1) Knifeblossom lies within influences of Snow Hell i.e. there is almost NOTHING worth plundering and has no value to the Tsaesci (except the human blood) whatsoever. Furthermore, the presence of the aggressive and unpredictable Snow Demons serves to deter any large scale invasion (modified from your point except that the human currently has no real strength to resist the Tsaesci upfront)
2) Knifeblossom is a "secret" stronghold. Nobody other than its inhabitants knows its position and this secret is guarded vehemently (intruders will be captured and quite possibly killed or kept imprisoned for life). I am just thinking, what about instead of just 1 fort, Knifeblossom is actually made up of some extremely tough to attck forts carved into cliffs and walls and such (similar to your original idea of "natural" fort) and linked by underground passages and even an underground "heart" city. Will have to work out some economic specialties like food and water sources, which can serve to make it stand out from just any city. The distributed sysem of forts with underground passages is also a safety mechanism : there are constructs in-place to collapse certain passages to "cut-off" a section/fort in case it gets discovered, as a safety-switch for protecting the secret location of the 'heart'. This could also be symbolic with the how the "flower" blooms outwards from the center and each small-fort/outpost/settlement is like a petal?
3) For historical background, we can have something like Old Knifeblossom Keep, which was the ruins of the original Knifeblossom Keep from some 5000 years ago. The keep was abandoned after the human empire was defeated (too obvious a target) and instead, its inhabitants stayed hidden away, while they eventually found a new place to build a safe haven. This also serves as a buffer such that whatever crap I decided to add to the lore regarding Knifeblossom, it has no direct relation with the current settlement since I would have been talking about the Old Knifeblossom Keep.
4) In my setting, one of the Tsaesci Lords (the fallen princess) is actually sympathetic towards the humans. She could be part of the hidden reason why Knifeblossom's location stayed secret from the rest of the Lords, although for mummery's sake, perhaps only the highest-ranking people in knifeblossom knows of this relationship, and she is referred to obliquely in another name/title (e.g. she can be regarded as a deity among the common human folks although nobody knew that their "deity" is actually one who would be considered their enemies..) sets the stage for some possible events that can be very "explosive"
Perhaps Cog can think of something?
5) As this may be the "last human stronghold", it could also be home to a faction whose aim is to free the human blood slaves in the Tsaesci Empire (though it will be tough as not only are the Tsaesci very powerful, but a lot of groups of bloodslaves already lost their sense of dignity/initiative and desire for freedom)