Columbia subject headings:
Bulgaria--Antiquities.
Fortification--Bulgaria.
Harvard subject headings:
Bulgaria--Antiquities.
Fortification--Bulgaria--Krŭn Region.
Krŭn Region (Bulgaria)--Antiquities.
Princeton subject headings:
Bulgaria--Antiquities.
Fortification--Bulgaria--Krŭn Region.
Krŭn Region (Bulgaria)--Antiquities.