-
barrier to the Turks,
might have been
formed south of the Danube. Delev,
Zlatnijat vek na bǎlgarskata kultura. Ivanova, "Introduction[permanent dead link]"...
- Baeva;
Bojan Dobrev (2006). "16 Bǎlgarskata dǎržava pri Car Simeon; 10
Zlatnijat vek na bǎlgarskata kultura".
storija i
civilizacija za 11. klas (in Bulgarian)...