Tower of London
It's hard to imagine the United Kingdom's capital city without the Tower of London, the formidable fortress erected by William the Conqueror in 1066. Dominating the British skyline along with Big Ben, St. Paul's Cathedral and the London Eye, the Tower is actually an entire castle that over the years has functioned as everything from a military barracks, execution site and prison to the home of England's crown jewels [source: PBS].
The tower's location along the Thames River was intended to strike a warning pose to would-be invaders during the 11th-century Norman conquest. These days, its near-sea level positioning makes the fortress vulnerable to climate change, according to researchers [sources: UNESCO, IOP Science].