Imperium Rex is a freeware turn-based strategy game similar to Empire. The main objective is to build up an empire by conquering all opponents and neutral cities. Up to three players can participate in a hot seat or play-by-email game. Each player typically starts with just one city and a few units, and needs to explore the world, gather resources, expand, research new technologies, trade on the global market, build armies and support infrastructure to eventually defeat the opposing player(s). Single-player games are limited to the practice mode where the goal is to conquer all neutral cities, which are not defenceless but will not expand or attack the player on their own.

The background story in Imperium Rex puts the player into the role of a sociologist who decided that empire is the best form of government, and set on a quest to unify humanity under a benevolent imperial rule. Unfortunately, others have also come to similar conclusions, and wish to create rival empires of their own. Many of the units and technologies available to the player are clearly futuristic (hover scouts, laser tanks, photon cannons etc.), suggesting that the events of the game take place sometime in the future.

Imperium Rex comes with several maps and a map editor, however there is no random map generator. Other options such as the number of cities and resources on the map can be adjusted by the player for each game session, and the position of these objects is randomized.