Located in Hamburg's nightlife hub of St. Pauli, the anti-aircraft bunker is one of the large concrete fortresses that can still be seen in this port city and across Germany and into which citizens were evacuated during 1940s air raids.

Long out of use, the large flak tower used to shoot down enemy planes has now been raised by five stories from 38 meters (124.67 feet) to 58 meters and relandscaped with a living exterior of plants over the past few years.

As of July 5, visitors to the St. Pauli Bunker can explore a roof garden with 4,700 plants, all while overlooking the city and other landmarks like its harbor, the nearby Millerntor football stadium and the Elbphilharmonie concert hall.

Among the most impressive new design elements is the large path, also lined with plants, shrubs and trees, which wraps around the outside of the building, leading visitors up to the rooftop gardens.

The building now houses an events space, restaurant, bar, cafe, and a hotel with 134 rooms, the Hotel Reverb, run by Hard Rock. A room for a weekend costs around 500 euros ($542) – relatively pricy by Hamburg standards.

On the inside, 80-year-old signposting continues to guide visitors "to the shelters" ("Zu den Schutzräumen"). The concept also includes a memorial and information center that explains the history of the bunker, which the Nazis had built in 1942 by forced laborers.

The listed building is not only one of Hamburg's most important memorials but has also been a hub for creatives and nightlife for years. The well-known Uebel & Gefahrlich nightclub has been using spaces in the bunker.

Thomas Matzen, the private investor who says he funded the green structure with around 60 million euros, sees it as a "beacon for climate adaptation and green urban planning far beyond Hamburg."

The green bunker will also be used to record and analyze data on the impact of green roofs and façades on temperatures, evaporation and rainwater for five years as part of a scientific project by the Technical University of Berlin.

Some critics have complained that the bunker isn't green enough yet, a criticism which the developers have rejected. "Of course, the final overall impression will only be created once the shrubs and climbing plants have reached their intended size," a spokesperson said. "And as every gardener knows, this simply takes time." The developers say the bunker can and will be replanted if necessary. "The St. Pauli bunker will be as green as planned."

According to the city, there are still around 650 bunkers in Hamburg today. These are spread across the entire city and serve as museums or climbing walls, among other purposes.