Built in the early 1940s on the orders of Adolf Hitler, Flakturm IV was designed to protect Hamburg from Allied aerial attacks. Almost indestructible, the formidable fortress was a symbol of Nazi military might and remained in place after the fall of the regime, a chilling reminder of the horrors of the conflict. Fast-forward to 2024 and Hamburg BUNKER, as it's now known, has been reborn as a hip urban sanctuary following an £84 million ($110m) revamp.

Click through the gallery to explore the history of this fascinating structure and get the lowdown on its truly remarkable reinvention...

Following Allied air raids on Berlin in 1940, Adolf Hitler ordered the construction of flakturms ('flak towers'). Masterminded by Friedrich Tamms, under the direction of the Fuhrer's chief architect Albert Speer, these heavily fortified structures were designed to be virtually indestructible. By 1943, the project had been extended to the port of Hamburg – a prime bombing target due to its crucial U-boat and oil industries. Flakturm IV, the fourth flak tower, was built across 1942 and 1943 by 2,400 forced labourers on the Heiligengeistfeld ('Holy Ghost Field') in the city's St Pauli district.

Totally self-sufficient, the near invincible bastion had its own electricity and water supply, as well as a fully functioning 95-bed hospital. To repel allied bombers, the building was armed with four monster anti-aircraft guns mounted on the roof. One of the largest bunkers ever constructed, Flakturm IV is a veritable behemoth, measuring 246 feet x 246 feet (75m x 75m) at the base and standing 125 feet (38m) tall. Its walls are 11 feet (3.5m) thick, while the roof was reinforced with 16 feet (5m) of concrete. Designed to accommodate 18,000 people, up to 25,000 Hamburgers (or 60,000 as per one account) sought shelter here during the devastating Allied attacks of July 1943.

After the war, a number of Flakturms were demolished as part of the denazification process. Flakturm IV was spared destruction since it was being used to shelter homeless Hamburgers. Moreover, the explosive force required to raze the structure would have caused severe damage to swathes of central Hamburg, so the decision was made to leave it intact. In 1946, the Axel Springer publishing house was set up in the building and in 1950, public broadcaster NWDR made history by transmitting West Germany's first-ever television images from the bunker. It also served as a women's refuge, a warehouse for teas and spices and an air raid shelter (again) when Cold War tensions escalated.

The building became a kreativbunker ('creative bunker') in 1956 when renowned fashion photographer F C Gundlach set up a studio in it. By 1975, 15 firms, mostly photography and advertising agencies, were working out of the building. By 1986, 25 companies were registered to it. That same year, the surrounding area was the scene of violent anti-nuclear protests. The issue of razing the structure arose again in 1989 when the Berlin Wall's fall heralded the end of the Cold War. The decision was made to keep it, partly due to the huge costs demolition would incur. While some saw the bunker as a blight on the skyline, others advocated its preservation as a memorial.

In the 1990s, the bunker was converted into a modern media and cultural centre. Investor Thomas Matzen acquired a 60-year lease on the building in 1993 for the equivalent of £4.7 million ($6.2m) in 2024 money. Nightclub J's opened in the building in 1999. The following year, the venue's VIP lounge was the scene of a grenade attack that seriously injured nine people, including Chris Harms, the frontman of dark rock band Lord of the Lost (who represented Germany at Eurovision in 2023). J's went bankrupt in 2001. In 2006, nightclub and concert venue Uebel & Gefahrlich ('Evil & Dangerous') opened here. Still going strong, the venue has hosted big names like Scooter and Billie Eilish.

In fact, the bunker and its surrounding area have long been synonymous with music and leisure. The Hamburger Dom fair has been held on the Heiligengeistfeld since 1893. The Heiligengeistfeld is also famous for being the location where photographer Astrid Kirchherr first snapped the Beatles in 1960 during their stint in Hamburg. The pics were taken a stone's throw from the bunker. The Beatles got their start in the St Pauli neighbourhood, playing in the Indra, Kaiserkeller, Bambi Kino and Top 10 clubs.

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In 2014, the bunker bolstered its reputation as a haven for quality music when the Resonanzraum opened in the building. Billed as the world's first chamber concert club, it's the home of the classical Ensemble Resonanz. Noted for its exceptional acoustics and casual atmosphere, the venue hosts a variety of musical events, including classical and jazz concerts, experimental projects and groundbreaking collaborations with electronic artists. The bunker is also home to a state-approved vocational school for pop music, a radio station and other music-related businesses.

Among the distinctive interior architectural features of the building are its four spiral staircases. During the Second World War, these staircases lacked railings, making them particularly dangerous during busy periods when Hamburgers risked their lives ascending them. No doubt terrified, children would hug the wall when climbing up according to local resident Ellen Schabacker.

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Proposals to revamp the bunker came thick and fast in the 21st century. Leaseholder Thomas Matzen rejected concepts ranging from a glass enclosure enveloping the entire structure to a 223-feet (68m) sphere on the roof and additional office floors, according to the Hamburger Abendblatt newspaper. Then in October 2013, entrepreneur Mathias Muller-Using and local resident Sonja Brier came up with the idea of greening the roof, to create a community sky garden. It would be accessed via a metal green 'mountain path' inspired by New York's High Line (pictured), the popular elevated park built on an old railroad track, and the Niteroi Contemporary Art Museum's ramp in Rio de Janeiro.

The idea of greening the bunker had in fact been floating around for quite some time. In 1992, Austrian artist Friedensreich Hundertwasser was commissioned by Hamburger Abendblatt to reimagine the former air raid shelter. The artist envisioned the bunker topped with trees, with his concept appearing in the newspaper on 2 April 1992. The artist is all about 'roof afforestation', which features heavily in his work, most notably in his Hundertwasserhaus ('Hundertwasser house') in Vienna.

Muller-Using roped in architect Michael Kuhn and engineer Tim Schierwater, and the team came up with a workable concept. A tiered five-storey pyramid structure was proposed for the roof of the bunker, comprising the rooftop garden, a 134-room hotel, restaurants, a cafe, an events and sports hall, and more. Matzen was sold on the idea. In the meantime, neighbourhood association Hilldegarden eV ('Hill Garden eV') was formed to facilitate public participation in the project, maintain some of the greenery and create a memorial to the victims of Nazism and the Second World War.

Matzen and co applied for planning permission for the project and it was given the green light in 2017. In terms of cost, an estimated £25 million ($33m) was budgeted for the revamp. It's not all been plain sailing since then, and the project hasn't been without its detractors. Following its approval, the greening had to be scaled back to meet strict fire safety regulations, attracting criticism from some who felt the original vision had been compromised. Other aspects of the project, including an additional structure on the roof that wasn't in the initial plans, have come under fire too.

Landschaftsarchitektur was appointed in collaboration with landscape designers Mark Krieger and Ingrid Gock to oversee the greening project, while planning and engineering firm phase10 Ingenieur was hired to take care of the construction of the rooftop extension. Various other companies have been involved in the project, including Buero 51, WTM, Argus, Larmkontor and Sumbi Ingenieure.

Work finally began on the facelift in mid-2019. The COVID pandemic presumably delayed construction, but by the end of 2021, the fifth floor of the pyramid-shaped extension had been completed. According to the Bunker St Pauli website, the bare structure weighs over 30,000 tonnes, equivalent to the weight of 60 fully loaded Airbus A380s. The load transfer of the extension was carried out via 16 humongous Geilinger columns on the uber-thick outer bunker walls.

The 335-step metal 'mountain path' began to take shape in 2021. Spanning 1,837 feet (560m) in total, the 16 foot-wide (5m) pathway starts at the main entrance and winds around the bunker's exterior, culminating in the tiered rooftop garden. It offers sweeping vistas of iconic Hamburg landmarks such as the Heinrich Hertz Tower, Elbphilharmonie, St Michael's Church and the city's harbour.

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In 2022, work commenced on the structure's interior, including the hotel, restaurants and other amenities. The entire project was completed earlier this year and opened to the public in July. Considerably over budget, the final bill came in at £84 million ($110m), including £51 million ($67m) for the construction of the extension, green roof and walkway.

The rooftop garden, which is exposed to the elements, calls for extremely hardy plants. Trees like field maple, apple and Scots pine, sourced from the Lorenz von Ehren nursery, were chosen for their resistance to drought, frost and wind. Shrubs, climbers, hedges and overhangs complement the tree selection. Many thousands of plants have been carefully selected to create this vibrant rooftop oasis.

In addition to their wider ecological benefits and aesthetic appeal, the green roof and facade act as a natural air conditioning system for the bunker. Over 60 sensors collect data on humidity, temperature and other conditions, which are studied to examine the greenery's effect on the microclimate. Admission is free and the garden is open from 9am to 9pm during the summer months, with reduced opening hours in the off-season. Capacity is limited to 900 people.

Hamburg-based RIMC Hotels and Resorts has partnered with Hard Rock Hotels to bring the REVERB by Hard Rock concept to the bunker, debuting the brand in Europe. Aimed at music fans, the 4-star hotel has 134 allergy-free rooms and suites. Designed to be environmentally friendly, the hotel and bunker complex are certified GreenSign businesses – achieving level four out of five for sustainability.

Music is at the crux of the hotel. As UK newspaper The Guardian has pointed out, the hotel makes nods to The Beatles. Like we've mentioned, the Fab Four cut their teeth in the surrounding area before they found international stardom. As a nice touch, the hotel rocks three affordable furnished guest apartments for music artists or scholarship holders that can be booked by Hamburg's cultural institutions.

The design aesthetic is modern and playful. The overall vibe is on the minimal side but with vibrant pops of colour, lots of neon signs and eclectic contemporary and street art to liven things up. Furnishings are sleek mid-century modern-esque, and exposed ductwork and pipes lend the hotel an edgy industrial feel. Music memorabilia and other unique objects add a quirky and personal touch.

Guests can choose between King, Double Queen and Roadie Bunk Rooms, all offering hip furnishings, fan-inspired artwork, a state-of-the-art entertainment system, work areas and bathrooms with high-end toiletries. Other room amenities include a fridge and smart TV. Many of the rooms feature breathtaking views of the Hamburg cityscape and harbour. Pictured here is one of the entry-level King suites.

Ideal for bands (or friends) on tour, the Roadie Bunk Room comes complete with two queen bunk beds and one queen bed, two private bathrooms, a karaoke machine, an oversized smart TV and more. Each room is decked out with a smart voice assistant that can access the REVERB City Guides, mood lighting, playlists, entertainment options, plus a whole lot more besides. Rooms in the hotel start from £135 ($177) per night.

Aside from the hotel, the bunker extension houses the Rock Shop, where guests can snap up music-inspired clothing and merch. A mix of Hard Rock classic and seasonal collections is available, as well as a local Bunker collection and FC St Pauli merch. There's also a three-field hall in the complex that can fit 2,200 people. It's named after heroic resistance fighter Georg Elser, who tried and failed to assassinate Hitler. The space is used by neighbourhood high-school students as a sports hall during the day and transforms into a concert and cultural venue after dark.

When it comes to dining, guests have several enticing options to choose from. La Sala (which is Spanish for 'the Living Room') is the bunker's flagship restaurant. The 170-seat eatery offers a variety of delectable specialities, and operates from early morning to late night, with breakfast in the AM, a buffet lunch and tea in the afternoon and tapas during the evening. The venue hosts daily live music acts and DJ sets, making for a lively atmosphere.

KAROundPAUL occupies the first three floors of the rooftop extension, with the venue's 125-seat bar inspired by the surrounding Karolinenviertel and St. Pauli districts. It is rapidly becoming the go-to place for drinks in the neighbourhood. The bar, which like La Sala features regular live music acts, has a grungy punk vibe, with skulls and tattoo-style motifs a-plenty. Note the extra-covetable glass skull ceiling lights and graffitied walls.

The KAROundPAUL restaurant, not yet open but coming soon, will be curated by Michelin-starred celebrity chef Frank Rosin. Guests will get to dine on international fusion cuisine in the expansive 77-seat space, and marvel at the panoramic views of the Hamburg skyline and harbour. KAROundPAUL will also feature a private dining area for more intimate gatherings.

The bunker's 94-seat Constant Grind cafe has been described as a coffee shop above the rooftops of the city. Located in one of the bunker's towers, it features a bakery rustling up mouthwatering pastries, including regional classics such as franzbrotchen, which is cinnamon roll heaven. Given Germany's reputation for exceptional baked goods, you just know a visit to the Constant Grind won't disappoint.

Completing the bunker's enticing foodie offerings is the Green Beanie. Located in the rooftop garden, this super-cute pop-up container offers a small but irresistible range of light meals and drinks, with a menu that changes with the seasons. It's the perfect spot to grab a hot chocolate and snack while admiring the luxuriant plants and 360-degree views of the city.

Of course, the bunker's past hasn't been sugar-coated and neighbourhood association Hilldegarden eV has created a memorial and information site on the ground floor to honour victims of the Nazi regime and Second World War. As a symbol of hope and perseverance, the Hamburg Bunker stands as a reminder that even the darkest chapters of history can give rise to something positive and life-affirming, without overlooking the painful memories of yesteryear.

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