At the hubward-by-turnwise edge of Ankh-Morpork rises a small but rather steep hill which causes a protrusion of the Wall as it surrounds the rare prominence on the flat silted plain. A single road (Mithering Heights/ Old Tump Hill Walk) winds in a spiral to the top, where there remains an ancient ruin, just the foundation of a great square building or wall. This is assumed to have been the castle of the long-ago Kings of Ankh. It would make sense to have had a castle here, as standard architectural practice is that no city wall exists in isolation: the city wall around the base of the Tump is depicted as larger and more strongly fortified, as uniquely it boasts a plethora of defensive barbicans and sub-towers. A castle here would also command the heights and be strategically significant; it offers a garrison point for the troops detailed to defends and patrol the walls; and due to the old city aqueduct running underneath (as well as the possibility of naturally occurring springs and spas) there is a guaranteed water-source in the event of siege.
The stone of the castle has long since gone to build other parts of the city, although residual dungeons and crypts are said to exist within the Tump itself.
For centuries, the Tump, as it's known, was left undeveloped despite the extreme crowding and real estate values in Ankh-Morpork. Recently, however, the need for such a height of land and the corporate ego of Reacher Gilt, et al, created the Tump Tower. This combines the offices and the home terminal of the Grand Trunk Semaphore Company.
In midsummer The Tump also hosts a horse fair. The winding road up to the top of the Tump is known as the Old Tump Hill Walk, and it begins at Mithering Heights, a road just behind Spa Lane and Tump Lane. The Old Tump Hill Walk meets Brookless Lane opposite the junction with Small-But-Delicious-Fishes Avenue.
It has given its name to a House of study at the Assassins' Guild school. The reasoning has not been explained, but it might be out of a desire to inculcate in the girls a resolution to be as sturdy and steadfast as an age-old citadel. Or else Reacher Gilt, in a move to inculcate respectability and gain influential friends, bunged the Guild School a very large donation to facilitate Education and get a new building named after himself or a cause associated with him.
The word "tump" on Roundworld has two meanings. The first is a small hill, but the second - primarily Southern US usage of the word - is to overturn. Often used with over, as in, "You're about to tump that thing over" or, "Is that wheelbarrow going to tump over?"
It could also be a fairly obvious reference to the business and media tycoon Donald Trump, who developed Trump Tower in New York.