Get the pressure treated sleepers as suggested. They are smaller (2.4m x 200mm x100mm) than the old railway sleepers but cleaner cut. Will need treating with something like sadolin classic.
To get the most out of them, I have arranged them three courses high - stacked on the thin side ie. 3x200mm. You will need to get a hold of some 18mm rebar - drill holes in sleepers to accomodate rebar. Have rebar cut in lengths to suit ie. cut so that they are long enough to be run through the sleepers AND driven into the ground to hold them in place. On the top course, I have only drilled a hole half way through this sleeper - so that rebar/hole isnt visible on the top finished surface.
I will be lining the inside of the sleeper 'wall' with plastic to keep damp away. Also, have base slightly raised with stones so that moisture can drain away and they're not sitting in the wet 24/7. I hope that this will be enough to keep them from rotting away and falling apart.
Theres a good bit of work in it between cutting to size, drilling, treating with sadolin, etc - but worth it in the long run (i hope).