Friends buried a trampoline like that (but all the way) and used railroad ties / 4X4s as the retaining wall. I looked into doing it with mine and when lumber was cheap it cost many times more than the trampoline cost so I left mine up in the air like God intended. If it is concrete and it goes all the way around, it cost more than the pool. And so did the deck. And the slide, and the upper deck/stairs. And ….. lol. It’s utterly awesome but I’d never spend that much with a temporary centerpiece.