Salem

So I realized too late that building near a cliff was a bad idea. I need to make walls, but there are two single tile little cliff triangles where the ground slopes down. I can't dump dirt on them or dig very easily, but I managed to make them vanish. But I still can't move over that area freely, I have to climb. do I need to fill in the valley next to my house? bringing the low side lvl to my house would take forever.

You dont need to level it completely, but you do need to add more soil to it. After dumping each load, try to dig at the top, eventually you will be able to.
Best of luck with it

Yes, It is quite bothersome, that cliffs vanish even when the terrain is still too steep to be passable.

You'll need to lower the height difference between the bottom and top of the cliff to 10 or less before you can walk over it. Just keep at it

Also, to dig down a cliff, it's usually best to dig one vertex behind the cliff, then use the leveling tool (CTRL+Left Click in Dig Mode) to lower the cliff.