Columbia subject headings:
Castles--Europe--History--Congresses.
Water-supply--Europe--History--Congresses.
Waterworks--Europe--History--Congresses.
Harvard subject headings:
Blankenheim (Euskirchen, Germany)--History--Congresses.
Castles--Europe--History--Congresses.
Castles--Europe--Water-supply--History--Congresses.
Water-supply--Europe--History--Congresses.
Waterworks--Europe--History--Congresses.
Princeton subject headings: