"aperture, doorway, egress, entranceway, entry, entryway, exit, gate, gateway, hatch, hatchway, ingress, opening, portal, postern" This is the way that dictionary.com lines up the synonyms for "portal". (I thought it interesting that "aperture" was the first one.) Doors and other walk through portals (gates and arches) have long fascinated photographers. Perhaps its the notion of "it stops here and starts again on the other side" or just that doors and arches and gates and what not are often very cool. As long as a door or portal is a main part of the photo, it works for this project. So, show us your doors, gates, arches and ... apertures.