by ELLIOT CARTER

What happens when you combine Nazi propaganda, brutalist architecture and practical national security problems? You get chunky, concrete buildings that last for decades as symbols of war.

Flak towers were one of the Third Reich’s answers to Allied air attacks during World War II. These absolutely massive towers sheltered anti-aircraft guns in German cities—and protected their ammunition from falling bombs.

Several of these beasts are still standing today in Germany and Austria. Huge amounts of reinforced concrete in the towers—some have walls 11-feet thick—complicated efforts to demolish them after the war.

The remaining towers pose something of a dilemma.

Destroying them is pretty much impossible owing to their proximity to urban spaces—and the sheer amount of explosives required.

Faced with this problem, the post-war government in Vienna converted one tower into an aquarium. The Austrian army still uses another one.

Another debate in Hamburg erupted in 2013 because of plans by urban design firm Interpol Architecture. The concept envisioned a pyramid garden on the roof of a war-era flak tower in the port city’s St. Pauli district.

The designers want to incorporate cafes, clubs and performance art spaces in and around the structure, which sits astride a modern soccer stadium. But many locals believe the old military fortification should remain untouched—as a symbol of the Nazi regime’s terror.