Change the comma after city to a period. (The first two sentences were spliced together with a comma.)
Add a space between the words nothing and left. (Don’t get so hung up on punctuation errors that you miss simple typos.)
Delete the comma after left. (Not every use of but, and, etc., requires a comma before it.)
Spell out numbers instead of using numerals: six, not 6. (This is fiction, not journalism; rules specific to the AP Stylebook don’t apply here.)
Notice that the last sentence has been rewritten. The people don’t make up six city blocks of anything; they just live in a settlement that fills that space. Also, I deleted survivor from the last sentence because it was already established in the previous one that these were the survivors.
It was once a flourishing major city. Now there is nothing left but the empty, decaying ruins. A few small groups from the old city survived. These people live in the settlement that fills roughly six city blocks.
(WordPress’ spell checker doesn’t like Stylebook written as one word. Neither do I, actually, but that is the official way it’s spelled in this instance, so it’s correct anyway. *shrug*)