Garden Retaining Wall

[hELP1]

I currently have a raised area to the garden with a sunken patio area. I wish to raise the sunken area to the same height for child safety. The sunken area is about 2.5 ft deep. Wwhat is the best material for the retaining wall.. block or concrete?? How deep should the foundations be ? and finally should I make it a double skin wall with some sort of hard core between. Any help would be appreciated..

 

[Bahco]

18 inches deep by 12 ins wide should be more than enough for your footings.
I used concrete blocks ( same size as breeze blocks) for the wall. Single skin should hold 2.5 ft of soil but put some pipes in at the bottom of the wall for drainage.