Columbia subject headings:
Harvard subject headings:
Castles--Germany--History--Congresses.
Fortification--Germany--History--Congresses.
Historic buildings--Germany--History--Congresses.
Military architecture--Germany--History--Congresses.
Princeton subject headings:
Castles--Germany--History--Congresses.
Fortification--Germany--History--Congresses.
Historic buildings--Germany--History--Congresses.
Military architecture--Germany--History--Congresses.