surrounding it because it seemed like a bad idea to have concrete blocks all around it. So I attached pressure treated lumber and corrugated metal sheets right to the trampoline legs and let that be my retaining wall.

That also allowed to plant grass right up to it.

We also dug in a drain under it that goes down about 10 ft with an auger we rented, put a pipe in the hole and filled it with rock.

One tip we got was to leave the trampoline a foot or so above the ground level to allow air to escape. If you bury it flush, the air compresses under the trampoline and you lose some of the bounce.

I wish I would have left it about 18" above so it is easier to mow under the edge.

Here are a couple of pics.