It sounds like you would have been better off if you had leveled the entire area first, installed your timbers, and then tamped it all down. Doing the timbers after the fact makes me worry that the soil inside them won't be compacted enough. Beware that there are a lot of timbers out there that are not fully "pressure" treated. Their heart wood may rot in 5-8 years, attract ants etc. and be a big mess. Make sure the stuff you use is the highest quality available. You may be better off just making it lawn and berm it off in a rounded slope.