Columbia subject headings:
Fortification--Italy--Lazio.
Fortification--Italy--Viterbo (Province)
Lazio (Italy)--Antiquities.
Military art and science--History--Medieval, 500-1500.
Viterbo (Italy : Province)--Antiquities.
Harvard subject headings:
Architecture, Medieval--Italy--Viterbo (Province)
City walls--Italy--Viterbo (Province)
Fortification--Italy--Viterbo (Province)
Viterbo (Italy : Province)--Buildings, structures, etc.
Princeton subject headings:
City walls--Italy--Viterbo (Province)--History--Congresses.
Fortification--Italy--Viterbo (Province)--History--Congresses.
Viterbo (Italy : Province)--Antiquities--Congresses.