Empire is a strategy and tactics war game, pitting you against the computer. The game is played on a computer generated map that contains land areas, sea areas, and cities. The object of the game is to eliminate the opponent by capturing cities and destroying the enemy forces. Cities once captured have production capability, and can produce units such as armies, fighters, destroyers, submarines, or carriers for offense or defense. Of course, the computer is trying to crush your forces and take over the world, as are you.
Early in the game, exploration is most predominant, followed later by planned offensives and holding actions, till eventual victory or defeat.
Warning, typical games can take as long as ten hours, and some people find the game addictive. The program keeps the game state in a disk file, and a single game usually takes multiple computer sessions.
Since you can store the game on some cloud service (e.g. iCloud) you may also continue a game with the iOS/iPadOS version of Empire (https://apps.apple.com/de/app/empire-mobile/id1465452819) and vice versa.