Not sure if this is helpful for the scale you're planning (and certainly not an ancient technique!), but I've built our 8 meter (internal) diameter earth roundhouse house in New Zealand with a living roof blending into a bank on the ground on one side. I kept the roundhouse section self-contained, on its own rubble trench foundation with the earth around it angled away from the foundations and a secondary French drain installed to move any water away. I then extended the roof out from the house to the ground, creating a deep, protected verandah between the roundhouse and the ground, and keeping any ground water well away from the earth walls. The house was relatively very cheap to build from local roundwood timber for the frame, and local, natural materials for the walls. The biggest costs were the reclaimed joinery, and the pond liner we used to cover the roof.
The original plan was to use the verandah space for firewood and other storage, but whilst building the framing we decided to keep a higher head height and infill the spaces with additional rooms. The living roof drains down to the ground, where I dug another trench with a French drain to take away any roof water, which means there's now very little water coming through the bank, so I plan to put a membrane down and then create larder/root cellar/ cool storage areas adjoining the bank, then other living spaces on the inside of that. This effectively moves any potential problems further away from the actual house.
The roundhouse is now nearly finished and habitable, and it's worked out really well. I'm in the process of editing and posting a series of videos on YouTube which you may find helpful:
The channel will cover rubble trenches, roundwood framing, reciprocating roof, straw bale, cob, cordwood, light earth, earth plaster, lime plaster, living (sedum) roof, and hopefully much more
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