The Antonine Wall is a stone and turf fortification built by the Romans across what is now the Central Belt of Scotland, between the Firth of Forth and the Firth of Clyde. Representing the northernmost frontier barrier of the Roman Empire, it spanned approximately 63 kilometres (39 miles) and was about 3 metres (10 feet) high and 5 metres (16 feet) wide. Security was bolstered by a deep ditch on the northern side. The barrier was the second of two "great walls" created by the Romans in Northern Britain.