1) Apartment security feature 2) Aperture of a sort 3) Door feature 4) Eyehole 5) Feature of a front door 6) Hole 7) Home safety device 8) Hotel room door feature 9) Keyhole 10) Many a door feature 11) Security device 12) Security feature 13) See-through device 14) Small hole 15) Spyhole 16) Vantage point 1) Eyelet 2) Opening 3) Spyhole 
A peephole is a small opening through which one may look. In a door, a peephole allows people inside the security of seeing outside without opening the door. Glass peepholes are often fitted with a fisheye lens to allow a wider field of view from the inside and little to no visibility from the outside. ...
• (n.) A hole, or crevice, through which one may peep without being discovered.
Peep'hole` noun A hole, or crevice, through which one may peep without being discovered. 
Small hole in the kiln for the potter to see what is going on inside.
[n] - a hole (in a door or an oven etc) through which you can peep
spyhole noun a hole (in a door or an oven etc) through which you can peep
Small hole in the kiln for the potter to see inside during firing without the kiln losing too much heat. 
A small observation hole in the wall or door of a kiln.