Hi! Long time fan. Quick question, what’s the difference between a castle and a citadel?

Hi! Glad you’ve been enjoying the work. 

Good question!

Citadel is a word that’s used in a slightly confusing fashion when it comes to fortification. For example, citadel can mean a fortress attached to a city (whereas a castle may or may not be attached to a city) that forms the inner defenses, to which an army could retreat to if the city walls fell. 

However, a citadel can also be used to describe a part of a castle: another layer of walls between the outer walls and the inner keep. This is, however, a more rare use of the term.