Mr Monkey jumped off the underground at Tower Hill, crossed the road, let Mr Rik buy an entrance ticket, then scampered down the side of the Tower to the entrance. The battlements on the closest wall are where artillery casements were built inside the walls around 1862. You go in through the Middle Tower because they've very carelessly knocked down the original front door.
The Tower of London is historic, royal, and a palace, so there's a section about it at the Historic Royal Palaces website.
Copyright Rik Shepherd and Mr Monkey.