I built this map from open data. Every point is one of 5,793 castles, fortresses, palaces and ruins across 138 countries: 2,591 castles, 1,180 fortresses, 934 palaces and 1,088 ruins. The density is the story. Europe blazes while most of the planet stays dark.
It is a curated atlas, not a complete inventory. Germany alone has an estimated 25,000 castle sites (According to some sources). A landmark makes the map when it has a real photo on Wikimedia Commons, a Wikipedia article and exact coordinates in Wikidata, balanced across countries so the map stays explorable. France leads with 351.
You can explore every one interactively at https://thecastlemap.com/ with photos, founding centuries and short stories, plus a daily Guess the Castle game. There are no ads or sign up, everything CC0 and free to download.
I highly recommend it
https://www.parks.vic.gov.au/places-to-see/parks/point-nepean-national-park/attractions/fort-nepean