What do we call a person who designs the architecture or armors of a medieval fantasy film?

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 ;)}

https://simple.wikipedia.org/wiki/Armorer

Movie studio armorer On a film crew, an armorer is the person responsible for all the weapons and firearms used during the production. Armorers work with the producer, director and property master to create a list of all the weapons that will be needed. Sometimes the prop master is also the armorer. Often they research the style of weapons used according to the period and the kind of movie. For specialty weapons the armorer consults with the production designer and the prop maker. The armorer is responsible to make sure all weapons are used comply with federal and local laws. For scenes outside the studio, armorers work with police departments to get the necessary permits. To get the weapons they need, armorers may rent or lease them from licensed armories. Many movie armorers also have their own collection of weapons they may use. Armorers explain safe weapons handling to actors and crews making certain all safety rules are met. It is their responsibility to make sure nobody is hurt while weapons are being used.

For creating architecture, this will most likely be an "Art Director", or "Production Director", based on whether the architecture needs to be designed/painted or whether it's an existing building being used. "Stage Directors" mandate the look of buildings at a close scale (for cast members to interact with and act in front of).

Architecture is presented in different ways (actual buildings, matt paintings, smaller buildings, manufactured stages etc., so the control of those is passed to the most relevant crew. The main director, of course, will maintain and dictate overall control of the look.