In Citadel, the atmospheric new horror flick by newcomer Irish director Ciaran Foy, what we don't know can really, really hurt us.

Set in a grim council estate inspired by Northside Dublin but could really be anywhere in northern Europe, we meet Tommy Cowley (the doe-eyed Aneurin Barnard) and his heavily pregnant wife, who are about to discover that the local kids are genuinely unholy terrors.

Foy is a wunderkind graduate of the National Film School of Ireland, and Citadel is his attention grabbing debut feature. The film won the Midnighter Audience Award at SXSW, the prestigious annual film conference held in Austin, Texas, and I'm here to tell you it is creepy as all hell.

What's scarier than a bunch of creepy children? A bunch of supernaturally creepy children in hoods, and Citadel has them in spades.

When Citadel opens Tommy and his wife live a quiet life together in a falling down tower block, until the day the pair are inexplicably attacked by a group of hooded young thugs that kill his wife and leave Tommy to raise his newborn daughter alone.

Not surprisingly he's left broken and reeling by this turn of events and develops extreme agoraphobia. After that Tommy hides out in his new flat, safe from all real and imagined threats as he undergoes intense therapy sessions aimed at bringing him back to normal life.

But we all know he's not going to find a normal life until he finds out what really happened to him, and why. The question Citadel asks is, how you can you hope to protect your own child when you've become scared of your own shadow?

Foy was born and raised in Dublin and in fact still lives there. He knows the terrain of urban Dublin he's writing about.

“Yeah, I grew up in Coolock,” he tells the Irish Voice. “One day I was the victim of a pretty vicious and unprovoked attack when I was 18 by a gang of hoodies. They beat me with a hammer and threatened me with a dirty syringe.

“The scariest thing about it was that they didn’t want anything, they didn’t take anything from me. I was on the way home from the UCI (cinema) in Coolock when it happened.”

That changed you? It had to have done? How do you restore your faith in life after a completely senseless attack with no motive and no aim?

“Yeah, it left me with a trauma that eventually became agoraphobia (Tommy also develops agoraphobia in the film). I think when something happens, no matter how tragic or terrifying, when you know the reason behind it, no matter how insane that reason may seem - you can eventually make peace with it,” he says.

But life doesn't always give you that tidy option. Senseless violence is as least as common and people who have endured it can find it very hard to come to terms with it afterwards.

“It’s when you don’t know the reasons why and how.  That’s the embodiment of pure terror. That's what I wanted to do with this film,” Foy explains.

When the gang of hoodies that killed Tommy's wife return, this time apparently intent on kidnapping his daughter, Tommy is torn between his paralyzing fear and his protective parental instincts. But was it his own attack in Dublin, or was he thinking of what could happen on any working class council estate?

“I feel horror films today explain too much. I wanted to explain nothing, because as I say that's the darkness. That’s the pure terror. Not knowing,” Foy says.

“But I eventually put some bread crumbs in there for an audience to dissect and hypothesize about after the film.”

Initially Dublin was Foy’s setting and ideal location. But because the Ballymun towers were demolished he had to look elsewhere. Glasgow turned out to be the perfect stand-in because it has a lot of abandoned tower blocks.

And what was the origin of these creepy supernatural kids in hoodies? Where do they come from and what do they want?

“I saw them as becoming this way because they had no guidance, no father figures. They're abandoned youth in way. A lost generation of lost kids,” Foy offers.

There's a political and social dimension to all that, and having growing up in a working class suburb Foy knows it. Social failure haunts every frame of Citadel. There's a breakdown in everything, social order, relationships, parenting, religion.

But Foy didn't want Citadel to just be about urban alienation, and he's not making sweeping statements about the kind of kids you can find knocking around tower blocks. Instead he's more interested in the society they reflect.

The metaphors keep building with the introduction of a rogue priest who knows the truth about the origins of the hoodie nightmare kids. He knows where they came from and he knows that he's implicated himself. So far, so timely.

Is Citadel in part a metaphor for haunted Ireland under the church's dominance? It hard not to draw that conclusion.

“Someone pointed that out back home. I honestly didn’t approach it that way,” Foy says.

“I was looking for all manner of fathers -- a reluctant father, a faithless father and absentee spiritual father. But it's really interesting that it can be seen that way.”

Foy knows how to have it all ways. It could be a metaphor for the sexual abuse crisis, and at times it is explicitly so, but he's after bigger answers.

“I initially wanted to take the usual cliches of the genre, which usually has a Catholic priest who has all of the answers to a supernatural threat.  My priest is a faithless priest with no answers to a non-supernatural threat.”

The impression that won't leave you is how nightmarish and claustrophobic the world of Citadel is. Foy knows how to create a sense of menace in every frame, and as Tommy Barnard is an edgy bag of nerves looking for a way out.

You'll be gripped from the opening scene by this unusually thoughtful psychological horror film, which is a welcome Irish answer to the exhausted Paranormal Activity franchise.

“So many movies -- not just horror films -- I feel nothing watching, whether it's sadness or humor or terror or catharsis. I think people will feel something watching Citadel,” Foy says.

“They seem to anyway, whether I show it in Europe or the U.S. or South Korea. It freaks people out, but ultimately for me it's a story of hope and redemption.

“So many horrors end on a cynical twist but I wanted there to be salvation at the end. I want people to know this is not stupid horror. It’s got something to say but is an entertaining 85 minutes at the same time!”

Citadel opens in limited release on Friday.

Catch the trailer for Citadel here:

Welsh actor Aneurin Barnard is the haunted young man at the center of Irish director Ciaran Foy's debut feature Citadel.