Written agreement with neighbors even without a notary?

However, your neighbors - if I remember correctly, with their shared cul-de-sac - are the ones situated higher up.

Yes, correct, they are the ones situated higher up, but we are the ones who have altered the terrain.

yes, correct, they are the higher ones, but we are the ones who changed the terrain.

L-walls at common boundaries have not only a neighborly-legal dimension (where one can certainly consider who is the terrain modifier and/or main initiator of the measure), but "also" a physical-technical one (which can hardly be disregarded), namely: that the base belongs under the load.