The Tower of London was begun in 1078 by William the Conqueror. Successive monarches added towers, walls and a huge moat over the centuries. It was here that traitors were brought to be executed, via the traitors gate by boat from the Thames, before being beheaded.
These days it is rare for visitors to be beheaded, and the crown jewels can be seen, along with the Royal Armouries. In summer the queues can be quite long.
Next to the Tower of London is Tower Bridge, which is considerably newer, being built in Victorian times. It is also open to the public, and houses a museum of the bridge, along with wonderful views from the overhead walkways and the fine victorian steam engines which until recently were still used to open the bridge to allow ships to pass. This bridge is the unual shape it is, as originally pedestrians has the right to cross the river by the upper walkways when the bridge was open for shipping.
Just downstream from Tower Bridge, on the north bank is St Katherines dock, which has some expensive restaurants and some fine old boats. Just downstream on the south bank is the design museum.