Caesar (27.1-27.2)

However, the most of the Barbarians who escaped at that time took refuge with their king in the city of Alesia. And while Caesar was besieging this city, which was thought to be impregnable by reason of the great size of its walls and the number of their defenders, there fell upon him from outside the city a peril too great for words to depict.

For all that was mightiest among the nations of Gaul assembled and came in arms to Alesia, three hundred thousand strong; and the number of fighting men inside the city was not less than a hundred and seventy thousand. Thus Caesar, caught between so large hostile forces and besieged there, was compelled to build two walls for his protection, one looking towards the city, and the other towards those who had come up to relieve it; he felt that if the two forces should unite his cause was wholly lost.