Forum Claudii Vallensium is the Roman Martigny beneath the modern Alpine town. The remains connect the amphitheatre, roads, foundations, and museum pieces into a larger picture of a settlement that mattered because of its position on routes through the Valais.
It is best approached as a cluster rather than a single ruin. Walk the Roman sites with the amphitheatre in mind, and Martigny starts to feel less like a transit town and more like a place that has been managing Alpine movement for two thousand years.