Explanation: During the Dacian campaigns, the Roman army found themselves facing the falx, a curious weapon capable of hooking around Roman shields to get at the juicy, fleshy Roman bits within. The Romans, not being fond of this, adopted a new kind of ridged helmet and (temporarily) 'manica' or arm guards, for the duration of the campaign.
Centurii-chan, the artist here, draws every historical representation or figure they do as anime girls, because why not