The Battle of Alesia took place in September, 52 BC around the Gallic oppidum of Alesia, a major town centre and hill fort of the Mandubii tribe... To guarantee a perfect blockade, Caesar ordered the construction of an encircling set of fortifications, called a circumvallation, around Alesia. The details are known from Caesar's Commentaries. About 18 kilometres of 4 metre high fortifications were constructed in about three weeks (Wikipedia). This line was followed inwards by two four-and-a-half metre wide ditches, also four-and-a-half metres deep. The farthest from the fortification was filled with water from the surrounding rivers. These fortifications were...