Columbia subject headings:
Castles--Europe--History--Congresses.
Fortification--Europe--History--Congresses.
Harvard subject headings:
Castles--Europe--History--Congresses.
Castles--Europe--History--Sources--Congresses.
Fortification--Europe--History--Congresses.
Fortification--Europe--History--Sources--Congresses.
Princeton subject headings:
Castles--Europe--History--Congresses.
Fortification--Europe--History--Congresses.