Let's not forget that tags should be relevant to the actual piece of art - if knights and/or castles are actually in the context of the listing, by all means include them, but search engines do penalize for "improper use". Imagine the hits if you were to tag with "sex"...
Misspellings and alternate spellings are very valuable I have found through the years, they have generated many hits for me. I have another shop with vintage books, tagged both with "armor" and "armour", etc.
Adding predominant colors/colours of the piece is also helpful for exposure on Etsy, especially in treasuries. Many Etsians like to keep their treasuries in one color/colour scheme, so "blue", "gray/grey", "red", etc. may get you into an otherwise unexpected feature spot.
Child's room, man cave, wall decor, office decor....
HTH
;-))