The famous ancient structure built by the Romans in Britain is Hadrian's Wall. Constructed around AD 122 under Emperor Hadrian, this fortification stretched across the north of England, marking the northern boundary of the Roman Empire. Designed to protect Roman Britain from potential invasions by tribes from the north, Hadrian's Wall featured fortifications, watch towers, and military forts. Today, it is a UNESCO World Heritage Site and a popular hiking destination, representing the historical significance of Roman engineering and the military presence in ancient Britain.