A strongpoint too costly to storm

The island need not be easy to resupply to be hard to seize. Any German encirclement would require forces to leave the Elbe and Kiel Canal approaches and move into the defended waters around Helgoland. Heavy guns, mines, torpedo boats, and perhaps submarines could exact a high price before troops reached the shore. Other heavily defended islands offered a similar deterrent: invasion was physically possible but strategically wasteful. Germany would have to risk ships and soldiers quickly enough to win before the Royal Navy relieved the garrison.