This achievement is unlocked during the 3rd mission of the first Germans campaign, "Rudolf I".
There are 6 armors you must collect. In order to do so, you have to destroy the 3 blacksmiths within the city. Each blacksmith will drop 2 armors. Their locations are:
1: All the way down in the southeast corner of the castle, tucked in the back.
2: Northeast part of the castle, it's right by the outer wall, to the left of the rightmost secondary objective.
3: Directly south of the breach you enter the wall from you can find the final blacksmith, bythe back wall.
As soon as you pick up the 6th and final armor the achievement should unlock.