Nijmegen (NL), August 23-24 2008

[jvrjenivs]

wouldnt the Romans have made a double layer of planking inside and outside?

I also asked that question, but a young Roman soldier explained to me that it was not necessary; the space behind the planks is filled with the sand from the ditch. It is really strong.

You certainly need planking on the inside as the sand from the ditch put a lot of outwards preasure on the walls, but this doesn't say why you wouldn't put a second layer of planing on the outside.