Lemon on raised garden bed

Charles cant spell says...
Citrus have a surface root system. A raised bed or mounded earth over the roots is a good idea.
The only issue i see is if you have soft soil in a high raised bed the tree may blow over if you have wind/heavy fruiting.

I dont think you would need a retaining wall but as always a picture tells a million words, so if you want a more confident answer attach a picture or two of the bed.