The Tower of London is an absolute must visit in London, there's not many cities in the world where you have a castle right in the middle! The Tower is officially known as His Majesty's Royal Palace and Fortress and was built in 1078 by William the Conqueror; it has served various purposes throughout history, including as a royal residence, prison and treasury.
You can do tours around the Castle to learn about the history, admire the acient castle and take a look at the Crown Jewels, which have been stored and displayed at the Tower of London since 1661!
Walk around the outside of the castle for some great photo oppertunities. From certain angles you can capture the acient tower in the foreground and the modern skyscrapers in the background!