White Tower (Tower of London)

The starting point: a Norman great tower meant to overawe a conquered city as much as to resist an assault. William I commissioned it after 1066, and at nearly 90 feet it could be seen from almost anywhere in London. Building took about 20 years, so William never saw it finished. The idea it shows is simple and powerful: stack everything a lord needs (stores, hall, chapel, living space) inside one massive masonry block that an attacker has to take floor by floor. Later entries on this list are largely attempts to move the fight outward, away from a single tower like this. It is still the core of the Tower of London, so the original Norman shell is easy to see on an ordinary visit.