As the other commenter said: Europe is filled to the brim with fortifications and castles. It's almost pointless to make a complete map - every second place will have one. So I agree with the approach of having a selection. But how to select? Here of course everybody will disagree - on one hand we surely have the obvious and well known tourist hotspots, but on the other hand basically everything else can fall under "hidden gems". Transylvania only has over 150 such fortified churches... what can you do with so much raw information?