(Warning: Extremely meandering thoughts here on changing perspectives on frontiers throughout history. Probably not interesting to most people.)
I think the correct overarching question for this course should be, "What would a Roman citizen [of various types, in various times and places] answer to questions about the edges of the Empire and what lies beyond them?" This question obviously extends naturally to later eras.
Obvious subpoints: What sort of sources are available? Surveys with random people in the street in classical Rome are notoriously hard to come by.
The course itself is focusing on the first 2 centuries AD, the period of full-speed Roman expansion. (Roughly from Augustus to Hadrian, with a bit of slop on the later side) My own background is more in Late Antiquity (3rd-6th centuries) through to the later Middle Ages, so it may be good to look at how those questions could be answered then.
- In the Middle Ages, there's a wealth of information. Especially after the Crusades opened up the Eastern frontier of Europe to travel (of one sort or another), there are a lot of narratives written by people who went travelling. This obviously has a selection bias - greengrocers typically weren't writing narratives. But these narratives were widely circulated in their own time, and there are substantial indirect sources as well: logs of trade, primary sources on urban life, and so on, indicating the extent to which international travel and new perspectives on travel infiltrated daily life. Phillips' The Medieval Expansion of Europe is an excellent analysis of this whole matter; it appears that there is enough information to synthesize a really coherent picture, and see how it flows into the Renaissance and Early Modern periods. I could babble about this subject for hours.
(Come to think of it, Umberto Eco's latest novel essentially is babbling about this for hours. It's basically a story built out of the travel literature of that period, especially John of Mandeville and similar mythographers) - In later Late Antiquity and the early Middle Ages (say, 7-9c, the "Dark Ages") there is much less evidence, but what evidence exists paints a pretty clear picture as well - namely, lines of communication had collapsed almost entirely, and it was almost impossible to travel even from one town to the next without a large military escort. Gregory of Tours is a good heads-up view on the earlier part of this period, but Cosmas Indicopleustes' Topografia Christiana (mid-6c) may be one of the most indicative things of all: his idea of the plan of the Earth (which is still the basis for flat-earthers today) and his notions of mapping are really those of someone who had not only never been out of a small area, but knew nobody who had, nor had even heard any reliable stories of anyone who had. The answer to questions about frontiers during this time period would have been terror and mostly incomprehension from all but the highest of nobility, and from those only a fairly localized picture of the world, maybe knowing about some neighboring duchies, but seeing anything beyond that as rapidly descending into the world of the mythical.
- Early Late Antiquity - say, from the time of troubles in the 3rd century up until Justinian's ill-conceived attempt to reunify the Roman Empire - is the really interesting case, a transition between the Antique perspective and the later perspective. The picture isn't fully clear in this period, and it's something I could babble about for hours, but really I want to get a more solid sense of perspectives in Antiquity in order to make a deeper analysis thereof.
- Imperial Rome, from Augustus up to slightly post-Hadrian, here simply "Antiquity" - this was the heyday of expansionism. There seems to be evidence that even the random schmuck in the street had heard of Parthia and the Danubian front, and had some notion of the extent of the Roman Empire. Some things about the perception of these boundaries was clear: they were targets for expansion, places we were going to conquer. Other things are less clear. In particular, what would people say about questions of what was beyond the borders? How much would the perspectives differ of people who lived in Rome and people who had actually been out to the frontiers? (This isn't an obvious question - the attitudes of the latter may have propagated widely into society)
One thing which may have significantly influenced this question is the Hellenistic view of the world as being comprised of cities (in the technical sense, i.e. πολ&epsilon&iotaς - cities with fora, markets, and certain other things, this being a recognized category in that time and place) with hinterlands in between. Towns, no matter how large, were really not much more than farmlands in the imagination of the time. Therefore it was possible for a soldier to come up to an actual boundary of Roman exploration, say going slightly beyond the Rhine or Danube, look off into the forests and so on and say that this was really the edge of the world, despite the fact that it was obviously possible to walk farther - there were simply no cities beyond that point. But what did individuals think about this?
I think the best ways to approach these issues will be to look for travel narratives. These may be unfortunately hard to find, but a few categories may be locatable: narratives by generals, histories written by people who had never been there, and itineraries. (The old substitute for maps - how to get from point A to point B. Dreadfully boring reading, but a potential mine of information)
Analyzing this question should give some good answers to how the Antique world evolved into the much more isolated, yet world-aware, Late Antique world. I've been of the opinion for some time that the real key characteristic of Late Antiquity is the changing relationship with the political Other (i.e., shifting definitions of Romanitas, dealing with barbarians as something other than fodder for conquest, not always being the ones in complete power, etc), and so a more careful examination of this question should be really interesting.
