The focal point for visitors to Nanjing is Zijinshan (Purple Mountain), which is so called because the rocks on top of the mountain look purplish red in the distance. It was also formerly known as Zhongshan (Bell Mountain) and Mount Jiang. The main site here is the mausoleum of China's great revolutionary forerunner, Dr Sun Yat-sen (1866-1925), which was built in 1926-1929 on the mountain's southern slopes. Now designated a national scenic spot and historical site, the mausoleum towers 73 metres at an altitude of 158 metres above sea level. At the foot of Purple Mountain is Xuanwu, a scenic lake surrounded by temples that date back to the pre-Qin period from around 221BC. Nanjing enjoyed its golden era under the Ming dynasty, and there are numerous remnants of the period around the city. The Ming Palace, now in ruins, was built by Emperor Hongwu in the 14th century, and it is said that it was a structure after which the Imperial Palace in Beijing was modelled. Today, all that remains of the once magnificent palace are five marble bridges known as Five Dragon Bridges, the old Wu Gate and the column bases of the palace buildings. The Ming city wall, built between 1366 and 1386, is the longest city wall ever built in the world: the wall is five storeys high and measures over 33 km long. The wall is notable for its irregular layout, which zigzags around the city's hills and rivers.