Building a Database of Historical Sites of Worship.

Interesting idea. I'd be looking at architectural history source books, or source websites. I guess that's an obvious idea Do you mean buildings/sites that were still standing in 1500, or construction begun or finished in 1500?

Check with the Vatican.
They can tell you where every Catholic church is, and when it was built.

Tourist boards. They are generally a wealth of information. Most countries will send you all kinds of free info.

Check with the mormons too. They seem to know a lot about other people's religions.

Here are links to all churches in Sweden. There you will be able to see when they were built.
The same for Denmark

http://sv.wikipedia.org/wiki/Alfabetisk_lista_%C3%B6ver_kyrkobyggnader_i_Sverige

http://www.reitoft.dk/kirker.html

Norway I only found a list of names of all churches in Norway. You have to look up every church individually.

http://www.tha.no/kirkerinorge-v2.htm

I have a family member who sent all our info to the mormons... and was quite wrong on several identies/points. I'm not saying not to check them, but am saying take all that as snippets of maybe.

Cathedrals could take centuries to build, or at least many years. Which is why I asked about the number, 1500.

The harder task here is for less noticible constructions of non-main 'religions' of their regions... or smaller shrines of main religions.

Medieval churches in Europe

Repertory of profiles of women's religious communities that existed between 400-1600 C.E.

Medieval chapels (in England)

Osso, no worries on the mormons, never used them myself but they seem to know a fair bit about people, don't know about the institutions...
I need a little bit of clarity on the subject, are you looking for places that were in use, or anything built for the purpose of, up until the 1500's
I was just in Ireland and I saw several ancient places, I doubt very much if they were in use, other than a curiosity during the 1500's. In Mexico and Guatemala, they seem to find buried temples every other day. Most of these would probably have been buried then too, but they were around...