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I'm not sure if I understand you question, but if you want a your 8 ft circle level, you could build a retaining wall at the "front" of the circle, backfill it with soil from the "rear" of the circle or you could remove the soil from the rear and build a retaining wall to keep the 18" from falling.
There are concrete wall blocks that are sold at a few homecenters that could work for you.
If you want the circle to follow the slope, you could mark the circle out on the slope,find some flexible edging material and fit the edging material into the soil, following the mark on the hillside. Dig out the material inside the circle to the depth for your gravel and sand from the surface of the hillside and then work upwards from the "front" of the circle to the "back". The edging could maintain the curve and by working from the front to the back, the pavers at the "back" of your circle would be supported by those in the front.