Columbia subject headings:
Harvard subject headings:
Fortification--Europe--History--19th century--Congresses.
Fortification--Germany--Cologne--History--19th century--Congresses.
Military architecture--Europe--History--19th century--Congresses.
Princeton subject headings:
Fortification--Europe--History--19th century--Congresses.
Fortification--Germany--Cologne--History--19th century--Congresses.
Military architecture--Europe--History--19th century--Congresses.