Movement 3D. Impede the character to come out of floor.

Hello! I wanted to raise a little problem. I created a 3D scene where the soil are individual cubes. The character walks over the cubes, like a board, but I want to impede the character to come out of this floor. Then there are other cubes on top of the floor made of cubes, and the character must be able to climb up to those cubes.

How I can do to prevent the character to leave this floor made of blocks? My first version I created some invisible colliders accompanying the character to avoid go out...