Patio on a slope

[robbbbbbbb]

I want to build a patio and the ground is on a slope. Maybe 15-20cm across a 4 metre patio. I know I have to dig down 15-20cm to get past the softer soil. My question is: Do I need to dig down further in the higher end so the ground is level before adding MOT1, or can I avoid the extra digging by levelling it at the point of adding MOT1, by just adding and compacting more of it at the lowest sloped end (which already has a retaining wall in place).