Many cat doors come with multifunctional locking mechanisms that allow you to leave the cat door completely open, completely locked, or something in between. For your cat’s safety, it’s generally best to lock the cat door at night so your cat can’t get out. If your cat is already outside, however, you should not lock the door – your cat should never be barred from reentering the home. Once your cat is in, you may want to lock the cat door to keep out feral cats and other animals.