Dunno if it goes so well in the Horror section, but I've seen other scripts so far away from the gender that managed to stay "Scary", why not call Nightmare a good fit too?
So, without spoiling the plot, here's what you're going to find in this one: an excellent story. Truly nice, you'll just have to be patient that's all, and let the pieces come together, just like in a puzzle because in the end, it will all worth your while.
But bear in mind, it is NOT you're typical horror, if you're looking for something scary, look elsewhere, if you want a night with cheesy and fun spooks, again, turn your head somewhere else, but if you require something a tad bit different, a look into a movie that reveals itself little by little, even annoying at times (because it does NOT rush whatsoever), i think you might hit JackPot with this one!
I like horror movies, pretty much all of them, not that new in the Asian spooky cinema, so i try not to be so harsh on them and wait to see a lot more before making a final statement.
Nightmare is a good add to the HORROR section, maybe more psychological, maybe more of a thriller too, but all in all, definitely a nice piece.
Then again, it might come down to everyone's opinion. So i'm looking forward into others, as mine is apparently the first.
Anyway, I do recommend this one, hope I was of some help.