Door knockers have been around for centuries, created for the pragmatic purpose of announcing the arrival of guests and, later, to make a visual statement. Some were even designed to ward off evil spirits.
Showing a grotesque human face (on a knocker or otherwise) at the door of a house has a longer history than you might think.
The theory behind the practice was that a god, or a scary face, will frighten off witches and malign spirits. That is also why gargoyles (essentially just water-spouts) also have caricatured faces: to scare off evil spirits from accessing the church by that route. There was of course no need for a face at the main door of a church: the holy water stoup by the entrance would put them off trying that road.
Other common good-luck motifs on doorknockers and (for similar reasons) on decorative horse-brasses are the crescent moon (and its derivative, the horseshoe), stars, suns (until the mid-20thC including the swastika, originally a sun-disc) and flowers – especially those regarded as having curative or magical properties. ?).
I usually do not go around photographing front doors but I looked through some of my building photos and cropped down a few images to post here. However, I notice that newer renovations to town house have either omitted knockers or have created a larger two-door entrance. I copied the following from a source that called itself Michael B. Maybe, I will gather more images as I walked along the streets of New York City and elsewhere.