Important crossroads of trade routes
Fords and bridges across major rivers
Below important mountain passes
In case of endemic conflict - well defended places
Places with good natural harbors
Sources of valuable raw materials (flint, gold, iron, star diamonds, soul cucumbers, whatever )



There's usually one route that's the best from point A to point B, sometimes natural barriers will mean that multiple routes from multiple points follow the same route for a certain distance - that's an obvious trade route calling out for settlements and points of control to develop.

Hope that helps!