Britain could have made Heligoland a formidable fortress

Could Britain have retained and defended Heligoland into the First World War?

Yes; concrete, guns, stores, and the island's physical resilience could make it extremely difficult to seize, although Britain historically transferred it to improve relations with Germany.

The same hard geology that later complicated demolition would support deep fortifications and powerful coastal batteries. That military potential contrasts with the political purpose of the transfer, which Britain intended as reassurance but Germany interpreted as recognition of its status.