Trading Gate
This is the gateway in the city wall of Ankh-Morpork, where Short Street leaves the city. The name is clarified in the updated street map accompanying the The Compleat Ankh-Morpork. It was hitherto the only city gate left un-named on the previously issued The Streets of Ankh-Morpork.
The revised and expanded city map contained in The Compleat Ankh-Morpork has a handy little panel illustrating how all roads lead to Ankh-Morpork, even if you happen to be leaving the city and walking in the opposite direction, In this case out of Trading Gate, where the road leads to Ankh-Morpork via Überwald.
At the time of The Colour of Magic, this gate may have been known as the Widdershin Gate. It may informally still be called that in the present day.