It can be crafted in your Crafting Menu (default key "Q") or at any Crafting Table in worlds created before R22 on September 16th 2015, after the recipe has been unlocked upon buying it. The blocks will appear in the crafting menu in the "build" section under the headline "medieval".
The Medieval Crest Wall was introduced on May 1, 2017 during the R41 update.
The block shows three different designs and has one empty side. Two sides show a shield with a diagonal fleur-de-lis design, two sides show two crossed unsheathed swords. The top of the blocks shows two swords behind a shield, combining the emblems of the sides. The bottom side is empty, showing a somewhat weathered "indented" square surrounded by a raised edge.
To craft a stack of 8 Medieval Crest Wall blocks, you will need
The block can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.