The hierarchy of production design, below the actual Director, of course, radiates outwards and downwards like a family tree with more branches and people the lower you go.
At the top is the Production Designer. Below that both Art Director and Costume Designer. In some structures costume would be subordinate to both Art & Production, though some Costume Designers move in a rarified atmosphere all of their own & are subordinate to no-one but the Director.
Armour would be in the purview of the Costume Designer. Though [s]he would be unlikely to need hands-on experience at actual manufacture. That would be farmed out to the appropriate trade-craft. Design, through modelling & mock-ups, to physical manufacture.
Buildings/architecture would be under the Art Director - though again sub-divided into different departments for the actual manufacture. Real-life locations, massive outdoor set construction, models, CGI etc could all be used in creating the final building seen in different aspects.
A lot of this tends to be covered by the all-encompassing moniker "Art Department" which might employ a thousand people in a big movie, from the guy who looks after the props to the one who hung the wallpaper for an interior.
To complicate this slightly - in the past the 'top job' was called Art Director, but was re-titled to Production Designer - the Oscars were re-categorised to reflect this - https://en.wikipedia.org/wiki/Academy_Award_for_Best_Production_Design
Costume design has remained the same throughout - https://en.wikipedia.org/wiki/Academy_Award_for_Best_Costume_Design
If you want to see just how many people and different job descriptions can be involved in the technical structure of a medieval/fantasy movie, try scrolling down below the cast list on IMDB - Lord of the Rings or for the even more adventurous, Game of Thrones (don't attempt this second one on dialup, it will take a loooong time to load;)
I used this image in a recent answer - it seems apropos to re-use it here. Below the line is a physical set construction. Knock on it & it sounds like wood (because it is wood) above the line is CGI.
I may be out by one floor, left or right. It's really impossible to be certain even though I've stood on this set I still cannot tell exactly where the join is ;)