York was founded in 71 AD when Cerialis and the Ninth Legion constructed a military fortress (castra) on flat ground above the River Ouse near its junction with the River Foss. The fortress was later rebuilt in stone, covered an area of 50 acres, and was inhabited by 6,000 soldiers. The earliest known mention of Eburacum by name is from a wooden stylus tablet from the Roman fortress of Vindolanda along Hadrian's Wall, dated to c. 95–104 AD, where it is called "Eburaci". Much of the Roman fortress lies under the foundations of York Minster, and excavations in the Minster's undercroft have revealed some of the original walls.