The Berlin Wall.
Berlin’s division and unification have influenced the city historically, culturally, economically, and architecturally. During the Cold War, the two Berlins built ideologically typical structures. The wall’s fortifications created a meandering gap through the city’s core. During the decades immediately after the reunification, many buildings were refurbished, and gaps were filled with new buildings, slowly densifying and spatially unifying Berlin.
There is a great outdoor museum on the wall worth a visit. Its design is multifunctional: it preserves the Berlin Wall’s components, creates a great public space, and serves as a place for documentation.