n 2003, art collector Christian Boros bought a 1942 air-raid bunker as a private museum and storage facility, and then decided—hey, why not?—to build a penthouse for his family atop it. Says architect Petra Petersson, “There were restrictions on what we could do, since the building is listed by cultural heritage, but we also wanted to restrict ourselves so as not to compete with the structure. We felt it would be natural to use raw concrete for the walls of the penthouse, as the bunker is built from concrete.”