This is the most intense, claustrophobic sci-fi flick I've seen in a long time. Imagine you're a room 14x14x14 feet. Each door leads to another room almost identical as to the one you're in now. The only distinguishing characteristics of each room are the color of the walls and seemingly random numbers printed in the passageways. I forgot to mention the booby traps.
Some rooms have booby traps.
Some traps will dice you, some will incinerate you, some will hollow out your skull with hydrochloric acid. They don't know each other, all they know is they need to get out. This could have easily become a one note movie, but Natali and his crew keep managing to through new wrenches into the gears and the characters grow and change in awesome, surprising ways. And one point I was wondering which character will die next and that I didn't want any of them to die because I liked all of them. This movie was almost perfect.