The Great Fire of 1666 was actually only one of many to hit the mediaeval city – indeed the Wikipedia article on the Great Fire also refers us to Early fires of London and the Second Great Fire of London (though this is really the Blitz.) But the 1666 event was indeed Great, and after it was all cleared up they set about building a tower to commemorate it. Sir Christopher Wren designed the Monument, and it’s quite a masterpiece: even today it’s the tallest free-standing stone structure in the world, with 311 steps up to the viewing gallery. It’s very well worth the climb!
Photographs here include the view up the stairs, views from the top, the commemorative notice at the base, and the carving showing King Charles II directing the rebuilding.
Here are links to The Official Monument website and the Wikipedia page about the Monument