According to public lecture of russian linguist Anrej Zaliznjak there were process of melting of old-church-slavic with old-russian which resulted in modern russian (language of Pushkin). It's very simplified description, but anyway. Old-church-slavic was old-bulgarian in it's core and it entered to Russia as a translation of Bible. It was "high" and "bookish" dialect, but it influenced a lot modern russian.
Let's look at some examples from lecture, old-church-slavic style vs old-russian style (with modern meanings, old meanings were the same):

глава (leader, chief) - голова (head of man or animal)
страна (country) - сторона (side)
страж (guardian) - сторож (keeper)
град (city) - город (city)

As you can note old-church-slavic-styled words sill exist, but their meaning is shifted to more "high-order" and "important" things (but note, that with exception of "град" they are usual, everyday-speech words).
This is true in some manner for "град" too. Despite of the same meaning with "город", word "град" has more "pompous" tone. "Град обреченный" has strong poetic intonation. It's good for something like "Thou shalt fall, doomed city!". We do not use "град" in everyday speech, so it tends to become outdated.