Arthur's first instinct: making sure Merlin is safe. Nothing else matters.
Arthur & Merlin | 2.13 "The Last Dragonlord"
Interestingly, merlin does the exact same: look at him, he doesn’t look at arthur in search of support like someone else may in such situation, Merlin can fight and protect what he can see. By putting himself behind Arthur and looking at everyone in front of him he is metaphorically pulling out his sword and putting himself in front of Arthur
Anyway, I always find it neat how merthur can protect each other in a way that make the other not feel like they can’t do the same
