אַרְמוֹן 'armon "citadel, palace" used generally of prominent buildings: citadels, castles, palaces etc. this word is especially frequent in Amos. Over a third of all biblical uses of 'armon are in this little book (Am 1:4, 7, 10, 12, 14; 2:2, 5; 3:9, 10, 11; 6:8), all but one in chapters 1-3 (chapters 1-2 contain the oracles against the nations).
From the cotexts in which Amos uses the word we may suspect that he views these "citadels" as somewhat foreign to Israel (only 4 uses of Judean or Israelite buildings but 8 of foreign ones). The citadel or fortified temple or palace became inevitably, not only the last stronghold for a city's defense against invaders, but also often a symbol of oppression against the city's poor.