How can I loot this stash in Crow's Perch?

While walking around the castle in Crow's Perch looking for interesting stuff, I found a hidden spot with a chest full of good stuff. After happily looting it, I turned to walk away, but scanned the area with my Senses just out of habit, and noticed this:

See this glowing stash? I really want to loot it as well, but for the life of me couldn't find a way.

I tried to:

• Jump over the wall - no luck, it's not possible.
• Break the wall with Aard - no effect.
• Burn the wall with Igni - no effect.
• Hit the wall with my fists - no effect.
• Jump like crazy near the stash to get the "loot" icon - success, but being in mid-air, I couldn't activate it by pressing "X".

Any other ideas?