Hadrian's wall has been a UNESCO World Heritage Site since 1987. English Heritage, a government organisation in charge of managing the historic environment of England, describes it as "the most important monument built by the Romans in Britain". It was a defensive fortification in northern England, and it marked the border of the Roman Empire when it was at its height in the 2nd century. It was the most heavily fortified border in the Empire. In addition to its role as a military fortification, it is thought that many of the gates through the wall served as customs posts, to allow trade and levy taxation.