The first half of the series is surprisingly good. Interesting locked room mysteries with Sato Koichi providing the humour as Serizawa sensei. I enjoyed the physics solution to the problems like how someone could commit a murder, leave the room and make it look different.
From episode 7, Kagi no Kakatta Heya starts going into trickery instead of physics. The ending to ep 7 was decent. Ep 8 had a lot of Serizawa humour but an ending I found boring. When the solution is trickery, it becomes just another regular whodunnit instead of a how was it done. Suffice to say, only finished watching because I had come so far with it.
The final story should not have been split into two episodes. What a bloody waste of time and way too many red herrings. Completely unnecessary. Its like they got Tamaki Hiroshi and wanted to milk his appearance but ultimately, he was just another regular villain. Its not like he had this elaborate scheme to get the cops to suspect Enomoto. He needed to be a last boss level villain and ended up just another regular one.
Kagi no Kakatta Heya is only as good as its locked room mysteries were interesting. There needed to be more to the chemistry/role of the three protagonists in order to keep it interesting. I think Toda Erika's role is unnecessary since they didn't do anything with her character. Watchable.