This altar – to the west of the Temple of Heaven – was the site of solemn imperial ceremonies and sacrificial offerings. Here you'll find the excellent Běijīng Ancient Architecture Museum, which informatively narrates the elements of traditional Chinese building techniques. The museum is spread over the four 15th-century halls which face one another across the large courtyard. Each features different exhibits, but the centrepiece is the magnificent Jupiter Hall (太岁殿; Tàisuì Diàn), with exquisite detail in its ceiling.
The museum offers the chance to brush up on your dǒugǒng (brackets) and sǔnmǎo (joints), and get the low-down on Běijīng’s courtyard houses, while eyeballing detailed models of stand-out temple halls and pagodas from across the land. There’s a great scale model of the old walled city and English captions throughout. On Wednesdays, the first 200 visitors get in free.
Glance at any pre-1949 map of Běijīng and you can gauge the massive scale of the altar, which was built in 1420. Today, many of the altar's original structures survive and make up a tranquil and little-visited constellation of relics.
It's a fair walk from the subway station so tacking on a short cab ride isn't a bad idea.