You can build up, I did. It's a LOT more expensive than digging down. I recommend that you have the outer wall of the build-up re-inforced with a retaining wall. Use stone/block, not railroad ties.

Here's a familiar picture of mine--you can see the rows of block. Rather than rock dust, we used QP--Quarry Product, similar to crushed stone but not washed of the dirt. Packs like iron.