In 817 the Danish Vikings, who were allied with the Slavs, tried a large-scale attack on Esesfelth castle, but failed. As a result, the Frankish border was transferred back to the Elbe and secured with the Hammaburg and Delbende Castle (which is mentioned in documents, but so far not precisely localized). Delbende Castle was built in 822; therefore, one can assume that during 817 and 822 the late Saxon Hammaburg I from the 8th century was expanded to the fort Hammaburg II, which was twice as big. From an archaeological point of view the conversion of Hammaburg I to Hammaburg II can only be dated rather imprecisely to "about 800". But in combination with the historical tradition the rebuilding of the fortification may have happened after 817. At this point, the Hammaburg resembled the shape that was first described in documents.