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Retaining walls and terracing of small garden area

I need to get a sloping section of our garden levelled off with retaining walls at the top and bottom and along the sides. The top wall is going to be around 1.5m high and the bottom wall about 1m high. The walls are around about 6M long I think. I assume the easiest / cheapest is timber, but open to any options. Access is good for the digger and equipment. Thanks! Chris.