That's one humdinger of a raised bed!!!!!
In regards to the wall, which direction is the bed facing?
Being as it looks like your in Minnesota, that heavy wall will work well for holding some heat. Building a frame for a tunnel would be pretty easy and will extend your growing season a good bit, I would think. Other's in your type of climate should be able to give you an idea of how that would work. You definitely have a nice setup to work with!
Check out your "first frost date" to see how long you have before then. A tunnel will extend the harvest. You're fast approaching "cool weather vegetables" plus you have to consider decreasing daylight hours.
Seems someone recently posted a thread on starting a garden now...cucumber, radish, greens (spinach, lettuce, kale, turnips, etc), green beans, peas, brocooli rabe, zucchini, and suggestions already given. Watermelon and corn *might* be a stretch for this time of the year.