Columbia subject headings:
Fortification--Africa, North--History--Congresses.
Fortification--Portugal--History--Congresses.
Fortification--Spain--History--Congresses.
Harvard subject headings:
Fortification--Africa, North--History--Congresses.
Fortification--Portugal--History--Congresses.
Fortification--Spain--History--Congresses.
Princeton subject headings:
Fortification--Africa, North--History--Congresses.
Fortification--Portugal--History--Congresses.
Fortification--Spain--History--Congresses.