Like other states, an empire maintains its political structure at least partly by coercion. Land-based empires (such as the Mongol Empire or the Persian Empire) tend to extend in a contiguous area; sea-borne empires, also known as thalassocracies (the Athenian, Portuguese and the British empires provide examples), may feature looser structures and more scattered territories.
Empire Christian Music Achaemenid Empire Hasmonean Kingdom Cyrus the Great The Roman Empire