The Phrygians... select a natural hillock, run a trench through the middle of it, dig passages, and extend the interior space as widely as the site a… - Vitruvius

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The Phrygians... select a natural hillock, run a trench through the middle of it, dig passages, and extend the interior space as widely as the site admits. Over it they build a pyramidal roof of logs fastened together, and this they cover with reeds and brushwood, heaping up very high mounds of earth above their dwellings. Thus their fashion in houses makes their winters very warm and their summers very cool.