Hadrian's Defense is a reference to the WALL built by the Roman Emperor Hadrian in 122 AD. It was a defensive fortification that stretched across Britain from coast to coast, and was intended to protect the Roman Empire from invasions by northern tribes. The WALL was constructed of stone and turf, and was around 80 miles long. The crossword solution for this term is 'WALL', which accurately describes the structure built by Hadrian.