Now, years ago I promised a review. I even *wrote* a review, and posted a pocket version of it. Now I think it's time to bring the full review to you all. FreeTrav, this is especially for you.
(As an aside, I need to put it up in segments; this is a shared family computer.)
Mercator, which was written by Paul Elliot, AKA Mithras, is a fan-made supplement for Traveller. It is set in the eastern half of the Roman Empire during the first and second centuries of the Gregorian Calendar, and aims to adapt the original version of the Traveller rules to a low-tech setting. To that end, Mercator includes Roman-style careers for character generation, combat, equipment (not just weapons and armor), sailing ships, economics and trading (with a sailing bent), low-tech encounters and a brief survey of the Eastern Empire.
Character generation covers the careers of Legion (citizen soldiers, SOC 6+), Auxiliaries (non-citizen soldiers, SOC 5-), Navy, Merchants, nobles, rogues, barbarians and the ubiquitous "other," which in these rules represents crafters, townsmen, slaves and the assorted characters which don't fall under the other careers. These careers follow the standard 2d6 method and start at 18, except for Barbarians which roll 1d6 for Education and start at 14. (In my opinion, the careers of Noble, Rogue, Other and maybe Merchant should begin at 14 as well, as humans usually begin their careers earlier in lower-tech areas - teenagers in this time were usually out working, and many teenage girls were married and/or mothers, more so than today.) The aging table is the one in CT Bk1, which is appropriate for the time period - once you made it past infancy you'd be fine 'til 60. Unless someone killed you, which was never unlikely.
The careers include a muster-out table, which no good Traveller career is complete without. Money is handled in silver denarii instead of credits, and the weapons are blades instead of guns. Indeed, like in sci-fi Traveller, Nobles and Merchants may aquire a ship - a wood sailing vessel without a mortgage.
One interesting change is that Nobles use the title that is the highest one that their families have held, which doesn't necessarily mean that the character holds it (I would not allow a PC to hold high titles without it being a political campaign), which is the other reason that I would begin Nobles at 14 - they had to undergo a course, the Cursus Honorum (Cursus!), which was designed to give them experience in how the Roman government worked. As opposed to commoners, who were supposed to let the First Families tell them what they *really* wanted to do.
There have been a few skill changes - for example, "Equestrian" - also a lower-noble Roman who isn't a senator but just rich - becomes "Riding" or Teamster, which is handling carts (which I would put together with Teamster when it comes up in Chargen - they're not the same, but a player making a legionary could pick Riding when he gets Teamster - once). There is also Blacksmith, Carpentry and Stonemason, which are specific and highly important subsets of Artisan (I would allow anyone who got a result of "Artisan" skill to pick these three, especially if playing an "Other"); "Artillery," which is ballistae and catapults, with Siege Engineer complementing them; meanwhile, Pilot and Small Craft refer to sailing vessels instead of starships (let me ask you what the difference between Star Ships and A Star's Hips is!).
[Here I must leave off. Others want this computer. I shall return!]