It's wonderfully refreshing to be able to review a film not in terms of how it copes in spite of its clearly modest budget but in terms of what it does as a narrative.
Treehouse is a deftly crafted exercise in claustrophobic suspense, where we don't need to see gore and big set pieces and instead are treated to the intense drama of a couple of kids trapped in a treehouse (for a large part of the movie) in the middle of nowhere, assailed by unknown evil entities, the identities of which we are given no real clue until the third act. The leads are fantastic, and the cinematography, sound and score are first rate. Carpenter fans will love this - a strange melange of assault on precinct 13, the goonies, lord of the flies, the descent and (arguably spoilerish) a few redneck horrors too.
There's very few flaws in the plot. For my money it could have been slightly shorter, could have eschewed a couple of obvious flashbacks that flag our hero's journey a little too obviously. But the atmosphere, the performances, and the photography more than make up for a few slight flaws in the script.
There's a lovely feel of old fairytale horror about it, particularly goldilocks and the three bears, which is dealt with in a wonderfully original way, and a lot of the real horror is dealt with second hand (over walkie talkies in a particularly effective emotional scene) and in revelation (without seeing the act itself) that allows us to really focus on the effect of the events on the main characters rather than cheap kills. It's maybe not a 'horror' flick, but is more horrifying than the slasher it could have been (thank god it's not in Wrong Turn territory!!!).
Despite a few minor gripes, this is a powerful and original film, which deserves not to be misunderstood.