When I saw the challenge 'Doorways' the first thought in my head was 'Number 10', one of the world's best known doors. My research on the net revealed , in a photographer's blog, that the number one is ever so slightly larger than the zero, and the zero, while being perfectly round on the outside is an uneven oval on the inner edge. So here is a small section of that very famous door, 10 Downing St, London, SW1A2AA, United Kingdom.
[and did you know that you cannot open the door to no. 10 from the outside - there is always someone on the inside to open the door - that is according to Stephen Fry on QI Margie]