Columbia subject headings:
Düsseldorf (Germany)--Defenses.
Düsseldorf (Germany)--History.
Military architecture--Germany--Düsseldorf.
Harvard subject headings:
Düsseldorf (Germany)--Buildings, structures, etc.
Düsseldorf (Germany)--History.
Fortification--Germany--Düsseldorf.
Princeton subject headings:
Düsseldorf (Germany)--Buildings, structures, etc.
Düsseldorf (Germany)--History.
Fortification--Germany--Düsseldorf.