The first thing to consider when making an old map in a period style is how maps at that time were made. Most Roman maps were not big on geographic accuracy, instead being based off of road networks. All that mattered was which city was connected to which other city, and the distances between 'em. This makes for a very weird looking map, and not the easiest to figure out.

In cases like this, it might be best to use some artistic license for the sake of understandability. It is possible for a geographically-accurate map to be present in a Roman setting; a nautical navigation map, for example. If you take this route, the best source for coastline shape &c. is definitely Ptolemy, who made a rather famous map of the world in the second century.

You shouldn't copy the style you'll see on versions of his map, though; the earliest copies of the map we have come from the late Middle Ages, and as such have a Mediaeval style.

So your best bet is to base the coastline, rivers, place names, and all that off of Ptolemy's map, but use the style of Roman road maps. Don't copy the style of the first one you see, though - there are quite a few Mediaeval and Renaissance copies of Roman road maps out there.


Hope this helps.