Columbia subject headings:
Fortification--Italy--Palermo (Province)--History.
Palermo (Italy : Province)--Buildings, structures, etc.
Towers--Italy--Palermo (Province)
Harvard subject headings:
Coast defenses--Italy--Palermo (Province)--History.
Fortification--Italy--Palermo (Province)--History.
Palermo (Italy : Province)--Buildings, structures, etc.
Watchtowers--Italy--Palermo (Province)
Princeton subject headings:
Fortification--Italy--Palermo (Province)--History.
Palermo (Italy : Province)--Buildings, structures, etc.
Towers--Italy--Palermo (Province).