Great project! Kudos to the designer.

Might need additional datasets though -- this one is lacking. only ~10 fortresses in France? LOL

The fortress / castle distinction is quite difficult and perhaps not useful? This map classifies the Crac (Krak) des Chevaliers[0], in modern-day Syria, as a "castle" but I would argue it's really a "fortress" (and in fact, one of the best preserved ancient fortresses in the world).

[0] https://en.wikipedia.org/wiki/Krak_des_Chevaliers