Al-Khali (also referred to as "Khali" in Pyramids), an ancient and prosperous city, is the current capital of Klatch and is located on the opposite shore of the Circle Sea to Ankh-Morpork and the Sto Plains, near the mouth of the Tsort River. First encountered in Sourcery, Al-Khali is famed for its Harem and Frescoes and is governed by the Seriph. It has a bazaar known as The Soak where many traders, merchants, slavers and thieves carry out their daily business.
Al-Khali is something of a mirror to Ankh-Morpork; it, too, boasts a large, unwashed population that will try and sell visitors anything, and a location similar to The Shades where those who wander in without armed guards are unlikely to wander out. Rincewind discovered this area during his visit to Klatch in Sourcery; his first clue that they had strayed into a danger zone was that no one had tried to sell them anything for five minutes.
Where Ankh-Morpork is famous for its smell, Al-Khali has its breeze; warm, sandy, blowing in from the desert, it is said to wear on visitors' sanities like a cheesegrater on the skin of a tomato, eventually seeming to rasp directly across their nerves.
The city boast a large temple to Offler, the principal deity of Klatch. In the heart of the city may be found the Square of 967 Delights (Khalians are meticulous about things that interest them) where hourly tours take visitors about the detailed Frescoes, although it has been suggested that unmarried women should not view the Frescoes and that wizards should be forbidden from seeing them -entirely-.
Al-Khali has a University, albeit a more mundane one, with a School of Medicine where John Lawn learnt the goal of medicine is to avoid actually killing people - a revolutionary and advanced concept for doctoring on the Disc.
Al-Khali's name, in classically Pratchettian multilayered style (thus, it is geographically appropriate, yet indisputably and ironically Jingo-esque) is derived from the Rub' al-Khali (Arabic: "the empty quarter"), one of the world's largest and most desolate sand deserts.
And on a third level: very intriguingly, a professional peer of Ian Stewart and Jack Cohen is Professor Jim Al-Khalili of the University of Surrey, one of Britain's foremost theoretical physicists. He has collaborated professionally with Cohen and Stewart, and one thing preventing him from being a British Sheldon Cooper is that he was born and brought up in Leeds and is a fervent fan of both Leeds United and Yorkshire County Cricket Club. It is difficult to envisage a Yorkshire version of Sheldon, and Jim has also had employment since the early 1990's as a science presenter for the BBC. The Al-Khali University (Mossy Lawn's alma mater) echoes his name....