A city like London which is 2 millennia old is naturally going to have many such 'situations'. The largest single devastation was probably not the Great Fire of London, but the Blitz, whereby one third of the entire city was levelled.....
Four losses off of the top of my head of a scale completely un-imaginable:
Euston Station
Once one of the biggest and oldest stations in London (built in 1837 - its older than most railways in the world), the station was not only an engineering triumph as one of the first central city termini, but for its architecture and its significance in termini construction. The later is especially true when it came to New York Penn (and Grand Central) which used the idea of what Euston had done with the Roman influences, but modelled itself upon the Baths of Caracalla. Both of these termini were built far later than Euston was, yet Euston and Penn both suffered the same fate: 1960's planners. Euston still exists in a horrible form - akin to an airport terminal from the 60's which relies heavily upon ramps and concrete. At least unlike Penn it actually penetrated by sunlight, but it still doesn't make up for the loss. Thankfully plans are underway to completely re-build the station, if only in glass and steel it should make the station look nicer.
Old Euston Station
Euston Station Today
Future Euston Station
Crystal Palace
Ever wondered where the 'World's Fair' started? Look no further than the Great Exhibition housed in the Crystal Palace. Possibly one of the greatest buildings ever built, it was built in 1851 and was 560m in length. Its original location was in Hyde Park, but it was then moved in 1854 to its site in Crystal Palace and significantly enlarged. A fire in 1936 destroyed the structure.
Before and After 1936 fire
Today
Whitehall Palace
Once the largest building in the world, it had 1,500 rooms and was originally the London residence of the monarchy. Built over several centuries it covered a whopping land area of 23 acres, practically all of what is now known as the Whitehall Central Government area (ie all the departments, NHS, Transport, etc...).
The original buildings dated back to 1049, but two fires in 1691 and 1668 destroyed pretty much the entire palace. Only one building (the 1622 Banquet Hall from which Charles I was executed) remains to this day from those fires, the rest was destroyed in a fire. The map below pretty much illustrates the immense size of the palace:
Alexandra Palace
Not really a palace in the sense of being a royal residence, but a 'palace of the people'. It was built in 1873 in North London and despite being destroyed in a horrific series of fires (1873 and 1980). Today it is a shell with a few parts remaining in action, yet it is mostly recognised for its history in tv. The worlds first regular public television service (by the BBC) was broadcast from here - the original transmitter tower sill remains.
The Fire
Today
One that really does stand out (but wasn't built in London) is the magnificant Fonthill Abbey, whcih was located several dozen miles west of London
Fonthill Abbey
It looks like a castle/abbey/cathedral but was infact someones house! Infact its probably the tallest personal residence ever built......
The man with the dream was Beckford - a rich man nontheless! Construction was interesting....it was completed, only to collapse immediately afterwards. Beckford then re-built the tower, but again it collapsed. For the third re-build it was finished completely in 1813. Yet again in 1825 the tower collapsed again....for the final time as all that remains are the foundations....
The front doors were 35 feet (10m) tall, Beckford lived alone and the tower rose to 90metres/300feet....
I kid you not, this was someones actual house - it even had a spire which unfortunately fell down as well!
