Hugel vs. earth berm for slopes

My property is on a steep hill, sloped towards the back. The rear yard is 50' wide by 40' deep with a gentle slope, plus about 6' of steep slope in the very back that drops down to a 2' tall concrete retaining wall and an alley. I'm figuring out what to plant there and thinking about whether some kind of berm or hugel mound along the top of the retaining wall makes sense, to take advantage of water flowing down the hill. I'm in a climate with wet winters and dry summers so I'm a little concerned about anything I build sliding down into the alley when it's saturated with water during the fall. Ideas?

Tegan, does the wall have any kind of drainage tile that would drain water from upslope?
Hugels tend to hold water, I am not sure but would a hugel cause water to build up behind the wall and cause damage to it ?

Without detailed information on how the wall was designed and constructed, I would advise against adding any soil or hugels to the steep portion of the slope or the retaining wall as adding additional soil or anything that retains water in the steeply sloping area adds to the load on the wall and increases it's chance of failure. Typically residential retaining walls aren't that robust to start with. The steep slope would be best used growing deep rooted grasses/broadleaf perennials/shrubs to help hold the slope in place.
I would put berms/terraces/hugels on the gentle slope a few feet back from the start of the steep slope.
Knowing the steepness of the steep and the gentle slope would help people make more specific suggestions.

Attaching a picture. The far end is the steepest slope, and already has a thicket of plums that I'm probably going to prune and keep. The wall is 2' high, 18" thick concrete blocks, so I don't think it's going anywhere. I was thinking more building up a mound on top of the wall, not really behind it. But maybe it would be better to just try planting blueberries or something on the slope.

Also the first two trunks there are stumps of trees that have been cut down. There's just one remaining cedar-type tree in the row.

The blocks appear to be solid concrete which makes for a much more robust retaining wall than I usually see. I agree that the area of plum thicket is rather too steep to work with. Maybe you could build a series of small terraces on the remaining section and dig shallow trenches to hold the terrace wall in place and to keep it from sliding down the slope during heavy rains. I would be concerned that a hugel wouldn't stay in place during the rainy season. Of course you could put a mini hugel in behind each terrace wall.

I agree with Stephen. I think if you did a terrace about two feet wide, even with the top of the blocks (maybe even with a bit of slope away from the blocks back towards your yard),then do some hugel berms up the slope at the top you could hold onto quite a bit of your moisture. Just be careful not to make the sides of your hugel too steep above the terrace. It could be pretty cool with little sun scoops and undulations in the hugel portion.