Бѣлградъ (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 “Бѣлградъ”, step by step
Бѣлградъ traces back through two separate lines of ancestry.