28 doors - Leopold's Gate

Leopold's Gate is not that interesting a door until you see where it leads to - the ruins of Hawarden Castle. It's a medieval castle from around the 12th century and whose rebuilding was a contravention of the terms of the Treaty of Montgomery and so ignited a war of Welsh Independence. The castle is open to the public twice a year and from the top (a bit of a scary climb of a narrow keep) there are some beautiful views.