The rugged terrain of the Qinling and Daba Mountains provided many narrow passes which were easily defended. These played an important role early on, especially during the Three Kingdoms period (220–280 A.D.) when two of the three contending states were located at either end of the Road to Shu. The famous Sword Gate Pass was so impregnable during this era that a small Shu army was able to successfully hold off an enemy of 30,000 here. For most of the road's long history, however, the passes functioned as military checkpoints where soldiers collected taffifs and verified travel credentials. Many of the most strategic passes now have highways running through them. The Qing dynasty fort at Sword Gate Pass (below right) and the plank road which accessed it from the north were both destroyed by a highway built in 1935. A new fort with some planking has been reconstructed to replace it (below left) and the site remains a popular, although somewhat Disneyesque, Chinese tourist attraction. The plank road through another of the barrier passes, Stone Sheep Pass (right bottom), was also destroyed by highway construction, but the scenery of the gorge itself which is several miles long is still spectacular. The only pass to have remained virtually unchanged for centuries (right top) is so remote that it has been forgotten by everyone but the locals. It can only be accessed by a day-long trek over two mountains on Hike 14.