Milecastle 39, also known as Castle Nick, is a Roman fortification along Hadrian's Wall. Milecastle 39 was partly excavated by archaeologists in the 19th century, and more fully in the 1980s by conservation workers. The milecastle measures 19m long by 15.5m across with stone walls standing 1.75m high. The excavations in the mid-1980s revealed that barracks blocks that were initially built inside the milecastle had later been replaced by small individual buildings. The milecastle was occupied continuously until the fourth century AD.