Columbia subject headings:
Bunkers (Fortification)--Germany--Otterbach Region--History--20th century.
Fortification--Germany--Otterbach Region--History--20th century.
Fortification--Germany--Rhineland-Palatinate--History--20th century.
Germany--Defenses--History--20th century.
Siegfried Line (Germany)--History.
Harvard subject headings:
Bunkers (Fortification)--Germany--Otterbach Region--History--20th century.
Fortification--Germany--Otterbach Region--History--20th century.
Fortification--Germany--Rhineland-Palatinate--History--20th century.
Fortification--Germany--Rhineland-Palatinate--History.
Germany--Defenses--History--20th century.
Siegfried Line (Germany)--History.
Princeton subject headings: