Imperium is a turn-based strategy game, where you begin as a leader of a small space empire in a single solar system. Your goal is to expand your empire both by colonization and conquest, extend your lifespan with the drug "nostrum" found on certain planets, and survive politically - votes are held every 50 years (game turns), and if you lose, the game is over.
The game focuses on some aspects of empire management not often seen in games: You can select planet governors, fleet admirals and ambassadors to other empires from a pool of politicians. Each politician has a loyalty to you, competence and charisma. You can improve their loyalty by giving them money and nostrum (thus also prolonging the life of useful politicians), but if they are illoyal and highly competent, they can become dangerous, beating you at elections or even founding a new empire with the planet they govern.
Economy, while abstract, is important. You can subsidize planets, set tariffs for trade with other empires, or put them under embargo.
The military side consists of building ships and troops, and assigning them to fleets which can be moved via a simple command system ("MOVE TO x, INVADE y, COLONIZE z"), and will then perform their orders in sequence. Technology does not play a major part, but you can build larger and more powerful ships over time.
Combat is fully automated, and depends on the competence of the fleet commander, as well as strength of the fleet.
The user interface consists of windows-like dialogs in black/white, and a 3D-map of the galaxy.