It's not clear how the map is built and if it is possible to contribute content. Only in my area there are dozens, possibly hundreds of castles that do not show in the map. Every city and many towns have one. Several hilltops have a castle or fortress too.
Maybe you must harvest data from all the national Wikipedias, easier said than done.
Example the page for https://it.wikipedia.org/wiki/Castello_di_Chignolo_Po exists only in Italian and in the local dialect. I'm sure that there are plenty of castles like that around the world without an English page.