When people go to London, the two places – besides Primark! – that they have to visit are Big Ben and the Tower of London. But be careful – they are not monuments. They are places of interest, or more commonly, sights.
This is why we use the word ‘sightseeing’, because as tourists we ‘see’ lots of ‘sights’.
A monument is a special physical tribute to something, either an individual person or a group of people. So a statue is a monument, or a war memorial is a monument. It is something that remembers people in a positive way.
Castles, astronomical clocks, cathedrals, towers and bridges are all sights!