The shield shape isn’t actually a cube, although it’s hard to tell from that perspective. Here’s a pic from the tutorial page that shows the shape that I believe kronborge is going for:

kroneborge can you show an image with x-ray turned on and an angle that shows the top edge of the shield? Or, what are your thoughts about sharing the actual blender file? Might be easier to debug that way?
Are the vertices on the face of the shield all in the same plane?