This magnificent structure, designed by Sir Christopher Wren, went up between 1675 and 1710. The cathedral's predecessor also went up – though sadly that was in flames, thanks to the Great Fire of London, which is why Wren was brought in to replace it.
Events of great national importance have taken place here over the years: Charles and Diana wedding; funerals for dignitaries such as Lord Nelson, the Duke of Wellington and Sir Winston Churchill; and Jubilee celebrations for Queen Victoria, to name but a few.
Wonderful stone carvings, ancient relics and beautiful works of art can be found throughout the cathedral, so do spend some time exploring before you embark on the climb.
The ascent is quite a thrill - you actually pass through the inside of the dome, along some narrow walkways. If you're not comfortable in tight spaces, maybe this isn't for you.
On the way up, you'll first come to the Whispering Gallery. It's an ideal place to take a break and get your breath back. Whisper a few words towards the far wall and you'll hear them echo back, such are the remarkable acoustics.
A further 119 steps up and you'll find yourself at the Stone Gallery. This runs around the outside of the dome. The view from here is great, but a better one is just around the corner (or, more accurately, up some more steps).
Eighty-five metres up is the Golden Gallery, perched on top of the dome. From here see how many London landmarks you can spot. For starters there's Tower Bridge, with Canary Wharf further in the distance. Tate Modern's close by, and now there's the City of London's new tallest building, Heron Tower. And look, over there in the far distance, looking rather diminutive – it's the London Eye!