Originally, Dartmouth had a harbor upriver. It shifted as a result of metal processing and tin streaming, which silted up the former harbor regions and flushed millions of tons of deposits downstream. This would have been a strategically placed entrepot for the commerce in cross-channel tin exports.
Long since abandoned, the prehistoric path from South Zeal that crossed Dartmoor south-east to Dartmouth appears to have been a part of a much larger route that ran the whole length of the county, beginning at Bideford on the north coast. Sailors are believed to have used it; you would think that they would have been better off sailing around, but it's a lengthy detour with some hazardous shoreline. There were rest houses every eight to ten miles along the route; in fact, the track passed directly through a house, most likely one of the old rest houses.