The battle of Mons Badonicus: the ultimate of Arthur's twelve victories over the Saxons and at which Arthur 'carried the image of the Virgin' according to Nennius. A shield depiction is a likely interpretation of this statement.
Ambrosius Aurelianus, a “Roman” according to Gildas, successfully rallied and organised the Britons, enabling them to decisively defeat the Anglo Saxon confederation at Mons Badonicus, ending the Germanic advance for at least 40 years.
Unique among the territories of the Western Roman Empire in the century, Britain succeeded in holding back and even reversing the tide of Germanic conquest for nearly two centuries. It was the Age of Arthur!