Sinju was a small town in Cathay which was nested against the Great Wall of Cathay.
In 1289, Marco Polo's caravan – which included the First Doctor, Susan, Barbara, and Ian – stopped there on its way to the court of Kublai Khan. It was in Sinju that Marco Polo discovered that the First Doctor had hidden a TARDIS key from him. This lead to the confiscation of the key and the placing of a guard at the entrance to to the travellers' tent, following the departure of Marco's caravan from the town. (TV: "The Wall of Lies")