Columbia subject headings:
Architecture, Medieval--Italy--Siena.
Siena (Italy)--Buildings, structures, etc.
Towers--Italy--Siena.
Harvard subject headings:
Architecture, Medieval--Italy--Siena.
Siena (Italy)--Buildings, structures, etc.
Towers--Italy--Siena.
Princeton subject headings:
Architecture, Medieval--Italy--Siena.
Siena (Italy)--Buildings, structures, etc.
Towers--Italy--Siena.