I know absolutely nothing about XIV century metallurgy and uniform standards, but aren’t those helmets too shiny? Yes, steel-shmeel, all that. But the guy with a crossbow has literally a mirror reflection of the ladder on his helmet!
Inspection day must’ve been that morning :p
Likely just artistic license to allow the equipment to be drawn clearly. Realistically speaking, mid-assault in a siege like this, everyone would probably be covered in a film of dust, dirt, mud, and soot, simply from so many people moving and so much activity going on, not to mention the days spent encamped outside of the fortification itself.
Inspection day
Oh, that explains everything in the most satisfactory way.
I mean, if you’re a soldier back then, you would have a lot of ‘down’ time. Polishing your helmet would be a good way to look busy

“Fais attention, mon amour.”
Jason Isaacs looking mfer


