The back panel is made from plexiglass and features a Pet Safe Cat Door with a two lock. The flap opening is 5.75″ w X 5.75″ H. It is recommended to every customer. When in use, it will allow your kitty cat access to their Cat Solarium any time they want. It also keeps the outside weather from flowing into your house and insects outside. The back panel can easily be removed for cleaning. If you have a large cat, 15 lbs or more, you can order a large cat door.