Beijing 72-hour visa-free transit - is it permitted to visit Great Wall?

We are UK citizens (UK passports) and will be in Beijing for 65 hours, en route from Sydney to London. We plan to use the 72-hours visa-free facility. We understand that we must not leave the municipal area of Beijing - is this correct? If it is, can we go to see a section of the Great Wall, or is all of it outside the area we must keep within?