At first they set up forked stakes connected by twigs and covered these walls with mud. Others made walls of lumps of dried mud, covering them with r… - Vitruvius

The part which is nearest to the earth... is without knots and is "clear." But the upper part, on account of the great heat in it throws up branches into the air through the knots and this... is called "knotwood" because of its hardness and knottiness. The lowest part, after the tree is cut down and the sapwood of the same thrown away, is split up into four pieces and prepared for joiner's work, and so is called clearstock.

The towers themselves must be either round or polygonal. Square towers are sooner shattered by military engines, for the battering rams pound their angles to pieces but in the case of round towers they can do no harm being engaged as it were in driving wedges to their center.

Oak... lasts for an unlimited period when buried in underground structures. ...when exposed to moisture... it cannot take in liquid on account of its compactness, but, withdrawing from the moisture, it resists it and warps, thus making cracks.