Vindolanda was a Roman military town for three centuries and had nine forts during that time, the first one predating the building of Hadrian's Wall. Most of the forts appear to have been built for single auxiliary units, according to Roman Forts in Britain, by David John Breeze. The most complete fort is Housesteads, which was attached to the rear of Hadrian's Wall and faced east. The Archaeology of Britain: An Introduction From the Upper Palaeolithic to the Industrial Revolution, by John Hunter and Ian Ralston, says it probably contained 800-1000 men and was occupied from the time of Hadrian, in the second century, to the end of the fourth century. It had a stone wall with four gates, with roads from the gates leading from the gates into the fort.
The fort at Vindolanda was built during the Flavian era, late first century A.D,. when it was built of timber. It was later replaced with a stone structure. It is in the waterlogged timber that the precious, preserved correspondences have been found.
Letters -- not all the douments are letters -- are described as scored and folded wooden leaf tablets, forming diptyches, with the text normally written in two columns, the left column coming first and often extending beyond the fold, and then right.