Well, something odd seems to have happened to the previous posts here, so I'll give it a shot.

The Alps had been major connective thoroughfares between Italy and the rest of Europe for many centuries before the Romans claimed them for themselves under the early Empire. The quantity of Etruscan material on Hallstatt-era sites, for instance, suggests intensive trade both over the Alps and up the Rhone River valley in France.

On this earlier trade, you can see:

Frankenstein, S. and M. J. Rowlands. 1978. " The internal structure and regional context of Early Iron Age society in south-western Germany." Bulletin of the Institute of Archaeology 15: 73-112.

Arafat, K. and C. Morgan. 1994. "Athens, Etruria and the Heuneburg: mutual misconceptions in the study of Greek-barbarian relations." In Classical Greece: Ancient histories and modern archaeologies, ed. I. Morris. Cambridge: Cambridge University Press, 108-134.

Caesar stomped around the Alps for a while, but it was under Augustus that the Roman generals Drusus and Tiberius both campaigned extensively in the Alpine region and subjugated the native tribes, most famously documented in the Augustan victory monument at La Turbie (France) in 6 BC. The Roman conquest of the region allowed for settlement of the valleys, largely by Roman veterans - the city of Aosta in modern Italy was founded ca. 25 BC along the route through the Great St. Bernard Pass, connecting Italy to the Lake Geneva and the Swiss Plateau.

To the east, Chur (Switzerland) was founded ca. 15 BC along the route which, through the Septimer and Splugen Passes, linked Lake Como and Milan to the Alpine Rhine and eventually Lake Constance.

These two routes were linked on a vaguely E-W access through the St. Gotthard pass, near to where St. Martigny (Switzerland) was founded under Claudius (over an earlier Gallic settlement).

Claudius also constructed the Via Claudia Augusta from Verona (Italy) to Augsburg (Germany) via the Brenner Pass, a route which went through Bolzano and Innsbruck.

These were the main routes which linked Italy to the north, and each was well maintained and traversed - some even had tracks cut for wagon wheels (Gleisstrassen in German), which were used long after the Roman period. Towns were settled along the routes at first to monitor traffic and local activity, later to act as weigh stations for troops, merchants, and travelers.

The best source I can recommend is still Drack and Fellmann's 1990 Die Römer in der Schweiz which offers longer explanations of settlement history and routes. You may also find websites such as Orbis or The Digital Atlas of Roman and Medieval Civilization helpful for exploring these routes on maps.

Hope that helps!