Say what you will about its starvation production values, the Irish horror film "Citadel" (★★, R for disturbing and violent content) is relentless. It takes place in an urban landscape so bleak it makes Chernobyl look like a Caribbean getaway. Even before the credits, pregnant Joanne (Amy Shiels) is attacked in a derelict apartment building by adolescent thugs. Her boyfriend, Tommy (Aneurin Barnard), looks on in anguish, trapped in a balky elevator and unable to help. Soon he's fighting crippling post-traumatic shock while struggling to protect their newborn daughter. The feral gang members -- who may be literally subhuman -- circle ever closer. Or is it all Tommy's paranoia? Only a hostile local priest (James Cosmo) appears to know how to combat the creatures, but with his own sanity in question, he's not the steadiest ally. Director Ciaran Foy maintains vise-like pressure and a creepy, spooky ambience from the very first shot. It's in dire need of some variety in pace and tone, yet it's nightmarishly hard to shake.