Built pretty much the same thing about 5-years ago with reclaimed hardwood sleepers, laid them 4 high horizontally, overlapped like blocks. Was laying a patio too so they were on top of compacted type 1 aggregate.
Used metal fixing band at the back to tie them all together & sunk some vertical sleepers into concrete in front of the wall & bolted them to the horizontal sleepers.
I bought a cheep circular saw from Screwfix with the largest diameter blade & RPM, didn’t quite cut through the sleepers so they needed turning to cut from both sides. Cut surprisingly easily & only used the one blade & saw is still going strong. Didn’t want to risk using a chainsaw in case there was any metal in the sleepers.
Back filled with 125-80mm clean aggregate to allow for drainage.
Did it on my own, they were unbelievably heavy! 5-years on, still looking good.