A castle here means anything Wikipedia lists as a castle — which often includes fortresses, palaces, monasteries, and châteaux. The list also includes historicist creations like Neuschwanstein, but only when their main construction was completed before the 20th century.
All castles are sorted into tiers based on nothing but the author’s vibe and whim. Feel free to rearrange them however you personally see fit. Ultimately, the criteria were not historical significance or defensive capability, but how exciting a visit there would likely be.