One thing that stood out is that there are a lot of European Gothic cathedrals. I think we could do with limiting the examples. Pisa seems obvious as inclusion anyway and has significant Gothic aspects, and then a sole northern European example, as others will be included in European cities anyway (e.g. Paris), also other inclusion have Gothic elements (e.g. Aachen)
Aachen Cathedral – Y – Perhaps the most important site for understanding the role of religion in European politics and culture. Also impressive architectural importance and longevity.
Amiens Cathedral –Y/N Either this or Chartres as the sole example of N. European Gothic, I prefer Amiens, but will readily concede Chartres is regarded as the most important.
Assisi – Y – Perhaps the best site to illustrate the veneration of saints, also impressive frescos.
Bourges Cathedral – N Think Chartres or Amiens are better examples
Burgos Cathedral –N- Better examples of gothic elsewhere
Canterbury – N?- Open to persuasion as protestant sites will be limited, but there are better examples of gothic cathedrals.
Cathedral of St. James in Sibenik – N- Regional significance only
Chartres Cathedral – Y/N – Either this or Amiens, Chartres is probably the most important
Cologne Cathedral – N?- if a second north European gothic cathedral is needed I would put forward Cologne as it is perhaps the most complete Gothic edifice, and took so long to build that it is actually a different version of the style.
Durham Castle and Cathedral – Y/N- Either this or Speyer as an example of Romanesque cathedral, my preference is for Durham due to the setting and column decorations.
Echmiatsin and Zvartnots – Y- Oldest cathedral in the world, in the first country to formally adopt Christianity as national religion (also bonus for not being in Western Europe!)
Hildesheim Cathedral and Church - N - I think Speyer or Durham would be better Romanesque examples, perhaps this is based on being locked out of it on a very cold December night though.
Kiev –Y- perhaps the finest Orthodox cathedral, large historical significance
León Cathedral –Y?- Need to include non European examples, but would places like Mexico City cover this anyway?
Modena – N- not interesting enough
Notre-Dame Cathedral in Tournai –N – better examples elsewhere
Piazza del Duomo (Pisa) –Y- finest Romanesque and gothic architectural ensemble anywhere.
Reims – N?- I would give preference to Chartres or Amiens, open to persuasion that this one is better though no more than one of the three.
Roskilde Cathedral – N? – Not an especially interesting building, but does represent Protestantism
Speyer Cathedral –Y/N- Either this or Durham for Romanesque, Durham gets my vote.
Vatican City –Y- Think this one is a dead cert, various pinnacles of European art in a small area, massive religious significance
Westminster – N- The Houses of Parliament are of more relevance, and would get my nod on that, also technically not a cathedral but hey ho!