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In July 917, Aethelflaed launched her first offensive foray and selected the fortress at Derby as her target. At that time the local ruler had probably joined with the armies from Northampton and Leicester in a number of raids to attack Mercia. Aethelfled took advantage of the weakened burh, and successfully assaulted the town in July 917; the whole region subsequently being annexed into English Mercia.
The Danes may well have established their military headquarters on the former Roman fort of Derventio. This 6-acre (24,000 m2) rectangular fort would have given the burh the equivalent of c. 500 hides. The Vikings had camped at nearby Repton in 874, and had abandoned it a year later after suffering significantly from disease during their stay (leading to the discovery of a grave containing 245 bodies).