A giant spyhole has been cut in the first floor to reveal two levels at once.
From the time.com
By boring a spyhole into the door of the presidential box earlier that day, the assassin could check that his intended victim had made it to the play and observe the box's occupants.
From the en.wikipedia.org
Because miniature painting was the court art par excellence, a distillate of countless man-hours for people with infinite leisure, it provides a spyhole to the detail of this vanished culture.
From the time.com
More examples
Peephole: a hole (in a door or an oven etc) through which you can peep
The opening in a kiln wall or door through which cones may be viewed during firing. Can also act as a steam vent during the early stages of firing.