The higher ground level is 0.80m to 0.85m above the lower ground level. We would like to replace a retaining wall that is already there. We would then like to put a 1.8m fence on top of this wall. We would like to drill fence post holders into the top of the wall if possible, but want to check that is ok. How deep must the foundations be? How wide must the wall be? The present wall looks too thin to us. Do we need anything like ties? There are no drains etc in the area.