Columbia subject headings:
Architecture, Medieval--Germany--Eifel--Congresses.
Castles--Germany--Eifel--Congresses.
Fortification, Roman--Germany--Eifel--Congresses.
Fortification--Germany--Eifel--Congresses.
Ruined buildings--Germany--Eifel--Congresses.
Harvard subject headings:
Architecture, Medieval--Germany--Eifel--Congresses.
Castles--Germany--Eifel--Congresses.
Fortification, Roman--Germany--Eifel--Congresses.
Fortification--Germany--Eifel--Congresses.
Ruined buildings--Germany--Eifel--Congresses.
Princeton subject headings:
Architecture, Medieval--Germany--Eifel--Congresses.
Castles--Germany--Eifel--Congresses.
Fortification, Roman--Germany--Eifel--Congresses.
Fortification--Germany--Eifel--Congresses.
Ruined buildings--Germany--Eifel--Congresses.