Personally, I love the actors' acting and all of those emotions they show on screen. It feels like you really one of the people that live inside the apartment and fight with other people in order to survive. I love how the directors putting very small details on every setting.
I love the characters, the setting, and the style of how the director makes the movie. The monsters feel quite real too. Though some parts of the monsters look really fake. Basically, all the settings give the audiences feeling of cramped home. You may feel like at home, but it's quite uncomfortable and cramped, that makes you wanna run away.
One of the minus things from this series is the plot. It's a bit different from the webtoon version, it's still good tho. BUT, if in the webtoon version the character development was explained thoroughly, in the movie, it was only a brief moment to see those character development. In other way, what I'm trying to say is, the plot is too fast.
You can't even remember few of the character's name, because the plot is too fast. Also, there are quite a lot of poeple in the ground floor, which means you should remember at least their personalities. But in this series, I don't think their names, nor personalities are that important.
So... GO WATCH THIS MASTERPIECE AND BE TERRIFIED