I've always liked the ways that geography and transportation shape the locations of cities. I'd like ideas for geographic barriers that could be overcome by an infrastructure project that would make a secluded spot in the mountains suddenly become a major trade hub. The infrastructure project should be ambitious but feasible with Medieval technology.
One idea I was thinking was a project to render a Mountain Pass that had previously been impassable in the winter usable year round. What would be involved in such a project for people without explosives or power equipment?
Another thought was a canal. What geography would result in a relatively short canal in the wilderness having a huge impact?
Ideally I'd want a configuration of geography that would create opportunities for a lockless canal and a "Mountain Pass Improvement" project near each other in the same wilderness.
I'm visualizing a scenario where an ambitious King has to build a temporary village/work camp to complete a project in an isolated and nearly uninhabitable area, and suddenly a booming cities springs up when the project is complete. This is a scenario that happened a lot later in history when railroads were built but I'm trying to think up scenario where this would happen in a less technologically advanced world.
I realize long canals and mountain terracing existed in the Medieval world but I'm thinking of a project with a quicker payoff.