The Antonine Wall was built by the Romans across what is now known as the Central Belt of modern-day Scotland. The wall was 63 kilometres (39 miles), was about 3 metres (10 feet) high and 5 metres (16 feet) wide. Construction began in AD 142 and took 12 years to complete.
Nothing is visible from the ground of the fort, but crop marks from the fort can sometimes be seen from the sky.
Links: www.antoninewall.org, en.wikipedia.org