Isn't that the one Albrecht Dürer used to sign his works?
Yes, that's Dürer's monogram. It's both visually appealing and evocative, so it felt like a good design for shields.
Thanks guys! Indeed, Mantic skeletons are not the best. I hate the heads and shields the most; that's why I replaced them. This dusty-rusty paintjob can hide their other faults and allows me to accentuate a few focal points - their new GW skulls in particular. With these few tricks implemented, they can end up looking pretty decent. Plus, it's quite quick, so I can make a small skeleton army in no time. When the round is over maybe I'll do a step-by step tutorial.
The charging knights together with their backdrop make a scene that's pleasant to look at. I second Hammers' question on the scale of the models; my eye is untrained when it comes to historicals.