Empire, in the old tty single-player game form, is a direct ancestor of games like Civilization and Sid Meier's Alpha Centauri. Basic idea was to take over cities and make them produce more soldiers and war machines, eventually taking over all of the cities. It didn't offer much diplomacy or management, or even too much strategy - given enough armies, the computer could be smothered anyway.
The old character-based Empire still exists, I've even played it. These days, however, it's easier to get XConq and play the included Empire scenario (though you must remember Empire had square-based map and XConq uses hex map, but that's a minor detail). XConq also comes with about zillion more interesting games that offer more challenge...
And yes, Empire was addictive. SMAC quadruply so. =)