Leaning Tower of Pisa

Cesare Borgia asked the Doctor what he thought of the Leaning Tower of Pisa, who answered "[that] it looked fine to [him]" (TV: Time Heist)

In 2012, Brian Williams visited it with the Eleventh Doctor. (TV: Dinosaurs on a Spaceship)

Behind the scenes
The Doctor Who Annual 1970 feature A Matter of Gravity depicted Galileo Galilei conducting various experiments on gravity at the Leaning Tower of Pisa in 1589.