I started off with clay and shale in my garden spot. A friend, the first year I was gardening, came over and rototilled the spot. He was stunned anything grew! I've been steadily increasing the garden space, following the advice I gave above. I've had my triumphs and woes over the years. I guess making that spot in my yard a workable garden space is a triumph!

I, too, have clay soil and poor drainage. The only place I have for gardening is in a lowlying area. I usually have some standing water after a rain event, but there are ways to cope.

Amending the soil really helps, but I think it's more important to look at the drainage pattern, the direction in which the water runs off, higher to lower. Dig trenches around the perimeter of the garden to direct it there, hopefully in the direction of the french drains.

Rather than raised beds, I do most of my planting in raised rows, so that the working paths between them also serve as trenches. When I plant crops like squash, I plant them in actual raised hills.

Doing this, I've successfully managed to grow crops like onions, that hate wet feet [always on the garden's high side].

Until you can make changes, make raised rows with drainage in the paths,

Perhaps the retaining wall is holding back water from seeping to lower ground.