Бѣлъградъ (Old Church Slavonic) comes from Old Church Slavonic градъ, from Proto-Slavic gȏrdъ, from Proto-Balto-Slavic gardas, from Proto-Indo-European gʰerdʰ- — to enclose, to encircle; enclosure, fence, belt.
Ancestry of “Бѣлъградъ”, step by step
Бѣлъградъ traces back through two separate lines of ancestry.