History of the Peloponnesian War

As for the wall of the Peloponnesians, it was thus built. It consisted of a double circle, one towards Plataea and another outward in case of an assault from Athens. These two walls were distant one from the other about sixteen foot;

and that sixteen foot of space which was betwixt them was disposed and built into cabins for the watchmen, which was so joined and continued one to another that the whole appeared to be one thick wall with battlements on either side.