Mercator is a historical role-playing game adapted from the Classic Traveller rules. It provides modified rules to adapt the science fiction game into a game based on Ancient Rome.
Although Traveller concerns itself with interstellar travel, piracy, misadventure and dubious heroics along the starlanes, it is at its heart a simple generic system. It has to be, to cover tech levels ranging from stone age to an era of floating cities.
It is the game's unique trade and travel system, however, that is its real strength. Transferred to Earth's ancient world, Traveller should prove successful, capturing the spirit of sea travel, haggling dockside at Caesarea instead of Glisten, fleeing from customs boats off of Corinth instead of Rhylanor. Traders in the Roman empire criss-crossed the Mediterranean, keeping the imperial economy alive and thriving, middlemen, sailors, chancers, veterans all trying to make an almost honest living.
Mercator is, then, an attempt to transport Traveller's ethos and game systems almost directly to the Roman Empire in the 1st and 2nd centuries AD. The Third Imperium becomes the Roman Empire, Regina becomes Alexandria, and goods like dried fruits or fish sauce are traded instead of machine parts or radioactives.
Many of the plots, stories and adventure hooks written for interstellar Traveller still work handsomely for a Roman campaign. Scams, smuggling, theft, ambush, raids, piracy and so forth translate easily between universes.
This shouldn't be taken as an accurate historical RPG version of imperial Rome, but it isn't a magical fantasy, either. Mercator is supposed to be a game of adventure, and the Roman Empire is a well known and familiar world full of tropes, adventure seeds and above all inspiration!
The game is available as a free download from the Freelance Traveller website. It requires Traveller (Classic) Books 1,2,3,4 & 5 and Supplement 4. The modified rules are defined in the Mercator book but the game mechanics are borrowed from Traveller.