This was one of my favorite dramas. In my opinion, this was perfect and I watched that three times, even my mom watched that!
Let's start with the acting of the main leads. That was wonderful and now Lee Do Hyun and Go Min Si are some of my favorite actors of the new generation. Their characters and themselves are really different and the fact that they can play a role like this so well is amazing (I really enjoy Youth of May too). Song Kang is a really good actor but the type of character he played was not my favorite but he did a really good job! To be honest I was really impressed with the acting of everyone, even the support roles, and the children.
The plot was very interesting and the action scenes with Warriors by Imagine Dragons were so energetic. The monsters were really well made and realistic to the point that sometimes I was scared by them and the writers were really original with the choices of the features of the monsters.
This is the only drama that I rewatched and it feels like watching that for the first time every time, even a scene that I cried every time I rewatched that I cried but maybe it's only me because I cry very easily. Now I can't wait for the second season because I have so many questions that I want answers to and some theories that I want to be resolved. In the end, I really recommend this k-drama for all the people that want a really good horror drama and some different from the rom-com ones; and it's a really good starting for being a horror/thriller/crime dramas watchers as it did for me.
(Sorry if I made any writing mistake but I'm not a native English speaker)