Very disappointed in this series. I wish they should've stick with monsters like in season 1. Here are my issues/thoughts about each season. Spoilers ahead!
Season 1 - storyline was okay and intriguing. I just watched Happiness and I really liked that kind of theme where people are stuck in one place.
I hated how they killed most of the main characters or the useful characters. Yu ri, the grandpa, the guy in the wheelchair, the sword man (forgot his name), they could've survived and let the useless characters die. I felt bad for the slime that saved the little boy! I also hated how the real Sang Wook didn't get much screen time in the next season. He was a great character
Season 2 - They focused on new characters and somehow forgot the monsters. I do wish all 3 seasons revolved around monsters. Again, they killed Ji su. Why?! She should've survived as well. I dont care about the other characters from season 1, most of them were useless, so im glad they died in this season. The mom who turned into a monster in season 1, such a waste! They wasted her.
Season 3 - boring season like season 2. I hate the girl for turning yeon su a monster. I dont like that kid and his mother figure from season 1, but they made it this far and they should've lived. Yi kyeong, the real Sang Wook, and In Hwan should've made it til the end. They killed the important characters again.
Honestly this series was such a let down.