עִיר `ir "city" almost always a fortified place (unless otherwise specified e.g. 1 Sam 27:5 "cities of the fields" = country towns).

In Amos the word charts the fortunes of Israel in the book:

3:6 warning and disaster
4:6-8 famine & drought
5:3 military decimation
6:8 delivered up by God
7:17 (captured) place of Amaziah's wife's prostitution
9:14 rebuilt.