Retaining hillside for above ground pool

Hello,

I am in the process of installing a 30ft above ground pool. The topography of our yard is very hilly. We excavated a site which required us to cut into a hill and now we need to retain it. I am trying to figure out the most cost effective way to retain the hill since where it will be retained will be covered by a deck we are going to install around the pool.

The wall will be 65 ft and at its highest point will be 3.5 ft. It tapers from the middle at 3.5ft down to 1ft on the end of each side and is a half oval shape that sits back 2.5-3 ft from where the pool is going to sit.

We are looking for the cheapest way. Even considering railroad ties or using rock from our yard which is piled up from various projects. Any insight here would be great :)

Thanks! John