If you are referring to the fact that the novel is known as a "locked door murder mystery," where murder is committed under impossible circumstances, which makes the mystery almost impossible to solve, because there is no evidence, as the theory of the locked door mystery suggests, that an outside intruder, or murderer could have committed the murders.
For example, a victim is found murdered in a locked room, the windows are closed, there apparently was no intruder, so who committed the murder? This is the type of murder mystery that keeps the reader on the edge of his seat.
"After the guests start turning up dead, the mystery deepens. Tension mounts as the remaining guests attempt to discover the murderer’s identity before they are all killed."
The mystery and suspense is heightened because the reader cannot figure out how, or who is committing the murders. Therefore, when the whole plot is revealed, it is usually very surprising.