The BT Tower in London is a 177‑metre (581 ft) cylindrical communications tower in Fitzrovia, built between 1961 and 1964 and opened in 1965 as the Post Office Tower.It was Britain’s tallest building on completion and held that title until 1980, and for decades it functioned as a key node in the UK’s microwave radio network while also becoming a major, if unusual, tourist attraction. 
 
The tower was commissioned by the General Post Office (GPO), which then ran the UK’s telephone service, to cope with rapidly growing demand for long‑distance calls and television relays.  Laying more underground cables across London would have been prohibitively expensive and disruptive, so the GPO opted for a high tower carrying microwave aerials that could beam traffic line‑of‑sight to other towers around the country. 
 
A revolving restaurant on the 34th floor (“The Top of the Tower”) and observation decks drew huge crowds; by the early 1970s more than 4.5 million people had visited.  I also got a chance to visit recently, so cool.
 
Interesting facts
Under the Official Secrets Act, it was technically an offence in certain contexts to publish or disclose details about the tower’s function, and for a long time it did not appear on official maps.