Tower of Art
The oldest building in Ankh-Morpork and part of Unseen University it stands an impressive 800 feet tall. With 8,888 steps leading in a spiral up to the top, it follows that each step is roughly an inch high.
It is completely without windows and, due to several repairs over the centuries, it has a gnarled appearance, much like an ancient yew tree. Atop it sit several small turrets and crenellations and within its stones entire ecosystems have formed, including species (and no doubt phyla) unique to the Tower of Art. A lack of predators willing to climb the vast heights of the tower has meant that several species of insects, birds and small mammals have been able to evolve fairly unaffected by the outside world, except by the magical waste from UU, causing some very odd results in the evolutionary pathways of these animals that would make Darwin just simply give up.
It is now disused and most of the internal floors have crumbled away, leaving just the spiral staircase. Or at least part of it; Victor Tugelbend needed to use Holy Wood magic to create replacement stairs for just long enough. Occasionally it is the scene of incredible fights between Dungeon Dimensions creatures and heroes, more unwilling ones than traditional Barbarian heroes.
Interesting to note, assuming dimensions of the steps, 1.08 inches in height, and 20 inches at their widest point, with a height of 8 feet (96 inches) between one step and the one directly above it (to allow wizard, plus at least 2 feet of pointy hat), there are 88.88 steps per loop of the spiral staircase (making for 100 loops of the spiral). Multiply that by the 20 inches of step, and we conclude that the tower has a circumference of 1,777.78 inches (148.15 feet).
Assuming the tower is a perfect cylinder, it has a diameter of 565.88 inches (47.16 feet).
This means it takes up an area of 1746.78 square feet, and has a volume of 1,397,421.63 cubic feet, so it could hold 69,634,475.5 UK pints (1,338,040,285 US ounces) of beer, which I think would keep the students happy for a while.
Then again, in the Discworld universe there are buildings that are larger on the inside than the outside so the above calculations should be taken with a grain of salt, especially as the Archchancellor has, as of Unseen Academicals, half a mile of good trout-fishing stream in his room.