How would being located on a lake and river benefit both a fort and traiding post?

Answer 1

A position on a shore is usually easier to defend because people in a fort can see boats coming across the lake or down the river. On hills along the Rhine River in Germany, many fortified castles were built hundreds of years ago.

Trading posts relied on lakes and rivers to bring in goods from afar. It's much easier to transport goods over water than over land.