Columbia subject headings:
Fortification--Italy--Siena.
Gates--Italy--Siena.
Military architecture--Italy--Siena.
Harvard subject headings:
Fortification--Italy--Siena.
Gates--Italy--Siena.
Military architecture--Italy--Siena.
Princeton subject headings:
Castles--Italy--Siena--History.
City walls--Italy--Siena--History.
Fortification--Italy--Siena--History.
Gates--Italy--Siena--History.
Military architecture--Italy--Siena--History.
Siena (Italy)--Buildings, structures, etc.
Siena (Italy)--History, Military.