How do I take apart a wooden retaining wall?
If you have access to a sawz-all also called a Tiger saw or reciprocating saw you can cut the spikes. Pry the timbers one at a time to get a little separation. Using the longest metal cutting saw blades you can get in the 14 tooth per inch range saw thru the spike. The longer blade will allow you to cut the spike if it is on the far side of the timber. Depending on what type of spike was used you may be looking at using several packs of blades. The process may get tougher near the bottom if they anchored the timbers to the ground with rebar.