Typically horror series focus on ghosts, demons or other supernatural being. Humans can be horrifying too when they do atrocious things.

What I liked:

1) Acting - Standout performances by Im Siwan, Lee Dong Wook and Lee Jung Eun. LDW is the reason I watched this series. Damn he was horrifying = he was great in this role. This is my first time watching Im Siwan's drama and I quite enjoyed his performance.
2) Pacing - I like the 10 episode format. Series would be draggy if it was stretched to 16 episodes and I don't think viewers could cope with any more torturous scenes.
3) Mindf**** - It's not just physical harm, but as a viewer I also could feel the pyschological pain & twists faced by Jong Woo. I felt really bad for him when he was at wits end and could not get out of the hostel situation.
4) Torture - This drama is not for the faint-hearted. Some scenes show physical harm, which can lead to stress/discomfort.
5) Music - The Rose is one of my favourite Korean groups. I was pleasantly surprised to hear their song in this series. Woosung's voice is hauntingly beautiful for this song. The Vane's song is also good. Good that the soundtrack is geared towards rock!

What I disliked:
1) Supporting characters - They were predictably dispensable. Wish the residents were given more screen time so viewers could relate and feel something when they died.

Favourite scene:
When Jong Woo and Moon Jo had the conversation on the rooftop. No violence but the dialogue gave me chills.