3rd-person RPG movment on uneven terrain?

I am quite new and just moved from 3D game studio to unity.

I am doing a 3rd-person RPG (like Diablo style). The movement is basically click on ground and walk to there. The problem is my terrain is uneven. I know I can use get Terrain height and keep updating y-axis each frame. But it has 2 problems: 1. Can't prevent character to move to a hill(which I don't want it to able to climb) 2. Can't prevent character to fall down cliff(which I want it stop before falling down)

Hope someone could give a direction to come out a good movement solution.