Stravaganza, The Queen in the Iron Mask(2012)

Stravaganza: Isai no Hime

A mysterious warrior travels the kingdom of Auroria, battling monstrous creatures and seeking adventure. Few know she is secretly Auroria’s young and inexperienced masked monarch -- Queen Vivian. In a world where humans are not at the top of the food chain, the Queen in the Iron Mask must learn to use both her sword and her throne to protect her people from the dangers looming just beyond the city walls.

(Source: Udon Entertainment)

Note: Includes two extra chapters.