if you have to or just want to put this raised bed on a concete base,
you could place several lengths of the pipes used for septic systems or
french drains which are full of holes and wrap them in construction
cloth to keep soil from filtering into them and have them come out
from under the little wall you will build so the bed can drain. Like
,space a bottom block so the pipe can peak out and allow the water to
drain out. Also to hold te soil in the bed if no morter is used (I
would not use any morter) you can line the bed with builders
construction costuction paper. I did this on y terraces and it works
quite well.

I bought some so-called top soil then ammended it heavily with leaves
and various soils from the local supplers and composted cow pooh.

I had a retaining wall leaning back slighty on/into a sloped embankment
going from 1 cement block (8x8x16") to 6 high with only gravity (their
weight ) holding them and no problem for 20 years and still counting.

good luck.--lee/leo