First, a courtyard is something a building can have. It isn't a type of building. So, use, 'a building with a courtyard." If you are describing the characteristics of the building use the present tense, "It is a building with a courtyard." If it is no longer a building or no longer has a courtyard, say, "It was a building with a courtyard." Is the building or the courtyard still called 'Siheyuan' today? If so, use the present. If the word, 'Siheyuan' is not longer used, use the past tense, "It was called Siheyuan." You can combine the tenses correctly. For example, if the building with a courtyard is still exists but the name was used in the past and isn't used now, you can say, "It is a building with a courtyard that was called Siheyuan."

You can use 'is' if the building is still called Siheyuan. If it is now called something different you should use 'was'. Example: "Until recently, the capital of China was called Peking by British speakers, but now it is called Beijing.'