The communes were strong enough to confine German Emperor Frederick Barbarossa's Italian ambitions to the south, where he secured Sicily for his Hohenstaufen heirs by marrying his son into the Norman royal family.
Frederick Barbarossa did not advance farther north than Sicily.
What is inconsistent tends to be examined more closely and may be rejected as less credible, unless-so the argument runs-there is enough detail that readers can see it with their own eyes.