This achievement is called "The Ura" with the description "Stop the Siege on the Bastion"
It is story-related and cannot be missed. It will be the story achievement you get after the secret achievement "The Singer".
Roughly 3/4 of the way through the game, after the level "Burstone Quarry" you'll play through a level that features Ura attacking you at The Bastion. It's very obvious when you are playing this level as it is clearly The Bastion and you'll be fighting a lot of pale-skinned warriors with polearms and bows. As soon as you defeat the very last enemy, this achievement will unlock.
Full credit goes to "FishyOutOfWater" for pointing out that the achievement is given on the level after "Burstone Quarry". Thank you :D