The planet's wandering magnetic poles

One debate among English historians regards what stimulated urban development in the fledgling nation. King Alfred, who with his brother unified the nobility in the mid-800s, commissioned earthwork defenses in many areas of southern England after Viking attacks in the 860s. One big question: Were the cities encircled by those earthworks well developed before construction of the defenses, or did those fortifications provide the protection needed for small villages to grow into thriving cities?